I think they're supposed to do it to completion. It's an adaptive test, so that means that the questions get harder or easier depending on if you answer the questions correctly or incorrectly. My son always takes longer on this test than his classmates. I think because it covers things he hasn't seen before, so he's trying to figure out what the answer could be. Maybe explain to your son that if he doesn't know the answer, to just make his best guess.