When generating the video, Steve will decide the scene duration based on the script and the voiceover (if any) in the current scene.

You can adjust the duration by a couple of seconds. Trying to change it any more will not be possible as the buttons (+ and -) will be greyed out.

The reason is that if you try to reduce the duration too much, then Steve will not be able to play the text and the voiceover completely.

And, when you try to increase the duration way more, the scene will have a lot of seconds, which might create a lot of stale scenes.