The duration for a motion menu is the Properties window, when you have the menu selected (not one of it's elements; click on the menu in the project).
How was the thumbnail created? Was it created by linking a video button to a timeline? If so, the duration of the thumbnail is going to be the same as the whole menu. If your menu is animated (or has audio), the duration of the whole menu will determine how long the thumbnail will last, too.
If you created the thumbnail manually, say with a video background element, you just need to set the duration of the menu. It should automatically be the same length as your video file, but it sounds like you may still need to tweak that setting.