Once you have the elpased time in minutes, the following will format it to HH:MM. This code assumes a positive elpased time. If the computation can go backwards, let me know and I will modify the code appropriately.

DECLARE @ElapsedTime INT; -- Elpased Time in munutes

SET @ElapsedTime = 358; -- Test value

SELECT CAST(@ElapsedTime / 60 AS VARCHAR) + ':' + RIGHT('0' + CAST(@ElapsedTime % 60 AS VARCHAR),2) AS FormatTime;