You already know that a slide show in PowerPoint can run manually or automatically by using slide timings. If you choose the latter, however, how will you know the amount of time to set for each slide? PowerPoint will allow you to rehearse the presentation, manually advancing the slide when you decide and recording the amount of time spent on each slide for you. You rehearse by choosing Rehearse Timings under the Slide Show menu.

Click on the Forward button (first button) to advance to the next slide. When you are done, you'll receive the following message:

You can then accept the timings or choose No and re-rehearse.