Take up to as much time as you think it will take you to randomly guess on those problems which you have not answered. I'd think it would not take more than 5 minutes max to scroll through and randomly make selections. You don't get penalized for a false answer, so might as well take a shot at it.
I was reading somewhere that a random guess might actually be better than sitting and trying to figure out the problem, because you try to reason through the problem and by doing so eliminate viable answers. I'm not saying do this for all of them, but if you absolutely don't know the answer it's best just to guess.