Here you go
DECLARE @TempReport TABLE (RunTIMe VARCHAR(20))
INSERT @TempReport VALUES ('2:30')
INSERT @TempReport VALUES ('5:20')
INSERT @TempReport VALUES ('6:10')
SELECT SUM(CAST(REPLACE(RunTime, ':','.') AS float)) from @TempReport
If '.' has to replaced to ':' cast to varchar again and do a replace