Do you mean such as the following:

1) Project started on Thursday at 3:30 PM
2) Project finished Next Tuesday at 10:00 AM?
...how many (work) hours was project's duration?

How about:
1. (StartDay)@5:00 PM - (Startday)(Starttime)
2. Now add all intervening full days as 8 hours (assuming no OT)
3. Then take: (FinishDay)(FinishTime)-(FinishDay)@8:00 AM...
4. Add 'em all up...

This is similar to Hilmar's response but within the time frame of 9 to 5

HTH,