Open the PowerPoint presentation
Go to the slide you want to adjust
Click the slide thumbnail in the left pane
Open the Transitions tab
Find the Timing section
Uncheck On Mouse Click if needed
Check After
Enter the desired slide duration in seconds
Click Apply To All if you want every slide to use the same duration
Go to the Slide Show tab
Click Set Up Slide Show
Select Browsed at a kiosk or use Loop continuously until Esc
Click OK
Save the presentation
Run the slideshow to confirm the loop timing