Believe it or not, there is a studied answer to this, and we think it will help you decide scene duration must faster and with much more confidence.
The secret formula is:
1 second per every 5 words at a minimum.
Simply count the number of words on your scene, click the scene block from within your timeline and extend it to the number of seconds it should be using the formula above. You can count the second on the timeline ruler at the top.
No more guessing here!