Sonya offered good advice about using Q&A time well. It helps to show your team the judging rubric in advance so they will see what key points they need to cover if time runs short - then they can think on their feet. It's a great life skill!
Also you might recommend they practice their set up. Quick set up preserves presentation time.
We encourage our team to keep it to 5 minutes or less out of Gracious Professionalism to the judges and all the other teams at the tournament. If your team sees it that way they will understand the rationale for sticking to the time allowed.
Co-Coach of the Brick Beasts