If not an actual time limit, maybe a sound and popup message when the time would run out if it were the actual SAT.

If I were designing it, I might also include in the summary what the score would have been if they'd had to stop at the time limit, and how far over the time limit they went.  If they finish early, I might include in the summary that they might have benefited from using the remaining time to check their work.

I love that idea!