This isn't going to be a very satisfying answer, but here it is:
- You should guess on as many problems as you need to in order to finish the entire section within the time limit. The whole point of guessing is to get through the entire section without running out of time. If you could do this without guessing
at all, then you should do that. If you need to guess 10+ times to get through the section, you should do that instead. Either way, if you guess
less than you need to, you'll run out of time at the end and you'll get a lower score than you could have gotten if you'd guessed more.
- Guessing doesn't hurt your score very much, because your score isn't calculated from the number of problems you got right. However, there are two things you want to avoid: making a lot of guesses in a row, and guessing on easy problems (because you're running out of time.) It's always completely fine to guess on a hard problem. You should also spread your guesses out throughout the section - in other words, guess
before you need to, rather than waiting until you're already falling behind.