You should create a calendar table that has the beginning of each month in the range of interest - sort of like shown in the first query below. Once you have that, join your table to that table like shown in the second query. I am only showing the general scheme of how to approach it; you will need to tailor it to your precise needs. If you need help with that, post some consumable code(i.e., code that someone can copy and paste to run on their computer, like the first query that I posted below)
-- Query 1 Create a monthly calendar and populate it.
CREATE TABLE #calendar(Dt DATETIME);
WITH cte AS
SELECT CAST('20000101' AS DATETIME) AS Dt
SELECT DATEADD (mm, 1, Dt)
WHERE Dt < '20151231'
INSERT INTO #Calendar
OPTION (MAXRECURSION 0)
-- Query 2 join your table to the calendar table like shown below
WHEN DATEDIFF(mm,INSTALL_DATE,dt) BETWEEN 0 AND 12 THEN '0 - 12 Months'
WHEN DATEDIFF(mm,INSTALL_DATE,dt) BETWEEN 13 AND 29 THEN '13 - 29 Months'
WHEN DATEDIFF(mm,INSTALL_DATE,dt) BETWEEN 30 AND 36 THEN '30 - 36 Months'
-- other ranges here
INNER JOIN YourActualTable i ON i.Install_Date <= c.Dt;