The breaks are always 10 minutes long. If you complete a section with 30 minutes to go, you don't get to add 30 minutes to your break. You just get the 10 minutes.

Now, if you want, you can sit in the room with the screen on the last question and just... sit there... while you wait for the time to run out. But I wouldn't recommend it.

Once you commit on the last answer, the test will immediately ask if you want your 10 minute break. If you take it, you have to leave the room. If you don't take it, you start the next section immediately.

It should go without saying but I'll say it anyway: TAKE THE BREAK!