Click the Rehearse Timings button on the Formatting toolbar.
PowerPoint switches to Slide Show mode, displaying the first slide and the Rehearsal toolbar, and starts a timer.
Rehearse what you want to say while this slide is displayed. When you are ready to advance to the next slide, click the Next button on the Rehearsal toolbar.
You can use PowerPoint's Rehearsal feature to time exactly how long each slide is displayed during the course of the show. The Rehearsal feature records the timings and displays them along with the slides in Slide Sorter view mode.
When setting up timings for each slide, be sure to allow enough time for the audience to read everything on the slide, as well as view any animation or multimedia clips.
Pausing the Rehearsal
If you need to pause your slide show during rehearsal, click the Pause button on the Rehearsal toolbar.
Repeat step 4 for each slide in the presentation.
When the slide show is finished, click Yes to record the timings.
PowerPoint displays the timings beneath each slide.
To view or change the timings, click the Transition button to open the Slide Transition Task pane and view the Advance Slide settings.
Redoing the Timing
If you make a mistake in the recorded time for a slide during the rehearsal, click the Repeat button on the Rehearsal toolbar to reset the timer and start again.
You can adjust the slide's timings in the Slide Transition Task pane. Click the slide whose timing you want to alter in Slide Sorter view, then click the Up or Down arrows beneath the Automatically After check box.
To make all the slides use the same timings, click the Apply to All Slides button in the Transition Task pane.