Solution:

1) Select the 'View' menu and select 'Slide Show...'. (The Slide Show dialog box appears.)
2) Select the 'Rehearse New Timings' radio button in the 'Advance' group.
Rehearse New Timings
3) Click 'Show'. (The slide show begins, with the 'Rehearsal' dialog box displayed in the lower right corner of the screen. The timing begins as soon as the first slide appears.)
Rehearsal dialog box
4) When done viewing the first slide, do one of the following:

a) To accept the timing for the first slide and to move to the next slide or build point, click 'Forward' in the 'Rehearsal' dialog box.
NOTE: Clicking 'Forward' means the displayed time for the first time has been accepted and will be recorded.
Forward button
b) To rehearse the first slide again, click 'Repeat' in the 'Rehearsal' dialog box.
Repeat button
NOTE: This may change the timing for the first slide.
c) To change the timing of the first slide, type a different value in the timing box.

5) (Optional) To stop the slide rehearsal at any time, click 'Close' in the 'Rehearsal' dialog box.
Close button
6) Repeat steps 4) and 5) until every slide in the presentation has been rehearsed. (The rehearsal ends. Then a message appears displaying the total time for the slide show and asking whether to save the new timings and show them in 'Slide Sorter' view.)
Message
7) Do one of the following:

a) To save the new timings and display them in 'Slide Sorter' view, click 'Yes'. (The new slide timings are displayed below each slide in 'Slide Sorter' view.)
NOTE: In slide view, the timings are the timings recorded by the length of time each slide was viewed during the rehearsal (i.e., until 'Forward' was selected in the 'Rehearsal' dialog box).
b) To ignore the changes made during the slide show rehearsal, click 'No'.