In the Animation palette, do any of
the following to set the duration of the video or animation you
want to export or preview:
Drag the ends of work area bar.
Move the current-time indicator to the time or frame
you want. From the Animation palette menu, choose either Set Start
Of Work Area or Set End Of Work Area.
(Optional) To remove the portions of the timeline not
in the work area, choose Trim Document Duration To Work Area from
the Animation palette menu.