;WITH
[4] AS (SELECT n = 0 FROM (VALUES (0), (0), (0), (0)) d (n)),
[16] AS (SELECT n = 0 FROM [4] a, [4] b),
[256] AS (SELECT n = 0 FROM [16] a, [16] b),
[65536] AS (SELECT n = 0 FROM [256] a, [256] b),
iTally AS (SELECT n = ROW_NUMBER() OVER(ORDER BY (SELECT NULL))-1 FROM [65536] a, [16] b)
SELECT a.visit_date, a.vin, a.make, x.Hour_in_service
FROM #sampleA a
CROSS APPLY (
SELECT TOP (1 + DATEDIFF(hour, a.enter_datetime, a.depart_datetime))
Hour_in_service = DATEPART(HOUR,DATEADD(hour,n,a.enter_datetime))
FROM iTally t
) x
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden