Taking what is currently scheduled as a three hour exam which many students already leave after 2, and for which some have accommodations allowing them 4 hours, and just setting aside up to five hours for it for everyone, likely makes the exam a fairer test of knowledge (as opposed to a test of exam skills and pressured time management) for everyone.
Once you’ve answered all the problems, or completed an essay, additional time isn’t going to make your answers any better. So you can just get up and leave when you’re done.
I didn't because I'd use the extra time to go over my answers again looking for errors.