------------------------------------------
--Set up a test table
set nocount on
if object_id('tempdb..#t') > 0 drop table #t
create table #t (id int, dt datetime)
go
------------------------------------------
--populate with sample data for 2000 rows for about 100 customers over about 3 month period at random times
insert #t (id, dt)
select convert(int, ceiling(rand()*100))
       ,dateadd(hour, rand()*24*30*3, dateadd(month,-3,getdate()))
go 2000
--select * from #t order by 1,2

------------------------------------------
--SELECT statement

select t.id
       ,avg(datediff(hour, t.dt, ca.dt)) [AvghoursBetweenCustCalls]
       ,min(datediff(hour, t.dt, ca.dt)) [MinHoursBetweenCustCalls]
       ,max(datediff(hour, t.dt, ca.dt))[MaxHoursBetweenCustCalls]
       ,count(*) [CustCallCount]
from   #t t
cross apply (
       select top 1 dt
       from   #t
       where  id = t.id
       and    dt > t.dt
       order by dt
       ) ca
group by t.id
order by t.id

output:
id          AvghoursBetweenCustCalls MinHoursBetweenCustCalls MaxHoursBetweenCustCalls CustCallCount
----------- ------------------------ ------------------------ ------------------------ -------------
1           85                       4                        282                      22
2           148                      11                       412                      14
3           81                       1                        262                      24
4           114                      13                       327                      18
5           102                      3                        526                      20
6           117                      0                        456                      16
7           111                      9                        375                      17
8           106                      0                        346                      19
9           92                       1                        348                      19
10          107                      1                        342                      19
11          128                      6                        512                      16
12          155                      14                       399                      13
13          92                       1                        377                      22
14          227                      30                       645                      8
15          117                      0                        318                      16
16          149                      18                       396                      13
17          84                       0                        382                      25
18          96                       0                        238                      22
19          114                      1                        334                      15
20          110                      8                        371                      19
21          87                       1                        286                      22
22          87                       10                       409                      24
23          128                      12                       306                      12
24          79                       5                        203                      26
25          89                       8                        192                      18
26          101                      6                        415                      20
27          110                      14                       348                      18
28          95                       17                       275                      22
29          71                       1                        278                      29
30          131                      8                        436                      15
31          131                      3                        395                      16
32          80                       3                        270                      25
33          114                      1                        528                      18
34          79                       0                        292                      25
35          106                      8                        337                      20
36          153                      1                        621                      13
37          91                       4                        436                      21
38          80                       3                        236                      25
39          93                       8                        260                      19
40          155                      3                        680                      13
41          109                      47                       286                      19
42          106                      17                       223                      20
43          99                       8                        421                      20
44          107                      9                        375                      12
45          148                      8                        379                      14
46          85                       3                        374                      19
47          116                      7                        252                      17
48          96                       4                        492                      21
49          118                      2                        309                      17
50          104                      12                       306                      16
51          129                      13                       275                      13
52          142                      4                        388                      15
53          110                      5                        429                      17
54          107                      22                       296                      19
55          100                      5                        259                      21
56          91                       3                        462                      23
57          75                       3                        237                      25
58          65                       2                        207                      31
59          162                      2                        420                      13
60          99                       9                        314                      19
61          141                      13                       363                      14
62          76                       3                        304                      27
63          91                       6                        272                      22
64          120                      12                       360                      17
65          71                       4                        307                      29
66          88                       2                        335                      22
67          88                       4                        363                      22
68          250                      5                        423                      7
69          114                      7                        246                      18
70          82                       2                        425                      25
71          126                      9                        521                      16
72          123                      7                        394                      16
73          100                      2                        340                      19
74          89                       2                        308                      21
75          93                       3                        391                      21
76          103                      13                       290                      19
77          80                       2                        200                      25
78          145                      17                       484                      14
79          92                       1                        373                      21
80          79                       3                        327                      25
81          86                       0                        287                      21
82          153                      2                        651                      9
83          86                       1                        365                      23
84          102                      6                        334                      20
85          105                      0                        303                      20
86          177                      1                        371                      11
87          181                      30                       668                      11
88          93                       0                        548                      22
89          106                      2                        333                      18
90          114                      5                        275                      18
91          97                       6                        526                      22
92          128                      6                        414                      15
93          95                       8                        266                      20
94          91                       3                        305                      20
95          67                       1                        267                      28
96          97                       16                       410                      21
97          83                       1                        413                      25
98          96                       3                        375                      19
99          102                      19                       205                      13
100         121                      5                        279                      15