To change the duration of a drop zone movie:
Make sure the menu that includes the drop zone you want to modify is showing in the iDVD window.
With your pointer over the menu, press Command-I to open the Menu Info window.
The Loop Duration slider indicates the length of the longest movie that has been added to the selected menu, either to a drop zone or as the menu background.
Drag the Loop Duration slider to the number of seconds you want the drop zone movie to play, up to the length of the longest movie included in the menu.