Scene duration is automatically determined by the length of the text caption you've assigned to that scene.
In Storyboard View
To change your scene's duration, click the 'Change Duration' button in the lower-left corner of the scene.
Once clicked, you'll have the ability to increase or decrease the duration by the whole second by using the + and - options.
You can also type in your media length to the millisecond by clicking directly on the current duration.
In Scene View
To change your scene's duration in Scene View, double-click on the scene you'd like to edit.
The 'Scene duration' button is on the first option in the media editing toolbar.
Increase or decrease your scene's duration by the whole second by using the + and - options.
As in Storyboard view, you can also type in your media length to the millisecond by clicking directly on the current duration.