They are tough! The best score I've been getting on G-chem is 22. most of the time 18 or 19.
But you should consider the errors. sometimes the explanations to the question is correct but the answer is wrong.
I have found OC to be tough, GC to be pretty on as far as difficulty (with a few errors), bio to be on, QR to be good if not a little easy, and RC to be too easy even with a short timer. Overall seems like a great resource though.