If you haven't already put the slides in the timeline, check out this:
http://www.sonicfoundry.com/forums/ShowMessage.asp?Forum=12&MessageID=80113

If you have already put the slides in the timeline, and you haven't
adjusted the length yet (they're still all 5 seconds), you can select all the
slides, group them together, and stretch the entire group to the new
total length you want.

- zoom out to see the entire slide show on the timeline
- Edit, Editing Tools, Selection
- drag the selection tool over all the slides to select them
- right-mouse-button click on any of the selected slides, group, create new
- Edit, Editing Tools, Normal
- position the cursor at the end of the last slide and you will see a small box with a doubleheaded arrow
- hold the control key down and slide the cursor left to shorten or right to lengthen all the slides together

If you wish to work on any of the slides individually, you must ungroup
them first. right-mouse-button click, group, clear.