1. Create a new calendar - call it 7Days.
2. Edit the calendar so that Saturday and Sunday are working days with the same times as Monday through Friday.
3. Add a spare Duration column (Duration1).
4. Right click on the column and choose "Custom fields"
5. Select Formula.
6. Enter the formula below:

ProjDateDiff([Start],[Finish],"7Days")

The tasks need to be auto scheduled.