First, have you read section 4 of this article:
http://tinyurl.com/GMATTimeManagementHave you developed your 1-minute time sense? Have you practiced applying it to specific problems as described in that section? If not, that's where to start.
The decision of "when" to do this* is essentially made for you: if you're at the 1 minute mark on quant or CR and you don't know what you're doing, you're done trying to get it right. If it's possible to make an educated guess, switch to that. If not, guess randomly and move on.
If you're at the 1 minute mark on SC or RC and you aren't close to done with the problem (at least 1 answer eliminated), you're done. Guess and move on.
Literally just start at that per-question level, learning how to make better decisions in the moment while working on a specific problem. Once you start to master that, you can work up to managing your time across an entire section.
*Note: there's one other category: "guess and move on within 10-20 seconds on my biggest weaknesses." When this happens is entirely based on your own weaknesses. I'm going to guess pretty much immediately on any 3D geometry and combinatorics because I'm terrible at those. But you don't have the same exact strengths and weaknesses as me.
I'm also going to guess when I see something that combines too many "ugh" characteristics. It's got roman numerals! And 4 variables! And an inequality! And fractions! Nope. Not doing that. (An "ugh" characteristic is something that I don't particularly love, but I'll likely do the problem if there are only one or two. Give me too many ugh characteristics, though, and I'm out of there!)
What are your biggest weaknesses? What are your ugh characteristics? How good are you at "checking in" with yourself after 1 minute and making a good decision about whether to keep going?