If those times are in Excel time format, then you can just SUM them
and set the format of that cell as appropriate, maybe [mm]:ss - the
square brackets instructs Excel not to wrap the number of minutes at
60 into hours, so that 1 hour and 30 minutes would show as 90:00
instead of 1:30:00.
If the duration is in seconds, then you could have a formula like
this:
=SUM(S2:S1000)/60/60/24
with the cell formatted as time.
Hope this helps.
Pete