You can give yourself a little extra time on some questions, because you will answer some other questions faster than the average time. But you can't go too far.
So if you have a question that's supposed to average 2 minutes but takes you 2 minutes and 30 seconds, that's okay, since you'll also have others that you answer in 1m30s.
But if a question takes you 4 to 5 minutes, that's an indication that you don't really know how to do that question in a reasonable time, so the best response is to guess and move on before using up all that time (and brain energy).
This means that you have to:
(1) learn how to identify what you can and can't do in a reasonable amount of time
(2) have the presence of mind to recognize when you can't do it (vs. thinking to yourself, "If I just had a little more time, I'm sure I could figure it out...")
(3) have the discipline to cut yourself off, guess, and move on
Section 4 in part 2 of this article talks about how to train yourself to do this:
http://tinyurl.com/GMATTimeManagementYou should read that whole article and start doing what it says, but section 4 will be of particular importance for the issue that we're discussing right now. I also recommend reading this article:
http://tinyurl.com/executivereasoning(In fact, I recommend reading that last article frequently.)
Next, you want to make sure that you're studying in a way that allows you to learn to recognize more and more when new questions pop up. This article talks about how to do so:
http://tinyurl.com/2ndlevelofgmatPractice that on old problems first - ones that you've already done. There's a lot you could have learned from those problems that you didn't yet. When you start to get the hang of it, you can start to mix in new problems again. (But still analyze old ones, looking for things that you didn't learn the first time around but can spot now.)