Some people really love pacing benchmarks on both the quant and verbal sections ("at question #20 I should have 35 minutes left" sort of thing), but I'm not usually a fan of benchmarking, especially on verbal. If benchmarks help you to calm down and have a general sense of where you stand on the test, fine. But if you use benchmarks too rigidly on the verbal section, they'll probably cause more harm than good.
This might sound really obvious, but most verbal mistakes happen because you misread or misinterpreted something, for one reason or another. Maybe you just couldn't make sense of a certain sentence, or maybe you missed a modifier or a detail that changed the meaning in some subtle way. Or maybe you were going just a little bit too quickly, and that caused you to make a small error in your reading or interpretation. A very tiny lapse in focus can cause you to miss a question, and that's a (presumably deliberate) design feature of GMAT verbal questions.
I think that benchmarking can actually cause you to make more of those small reading errors on the GMAT. On CR and RC questions, a huge proportion of your time will be spent reading and digesting the passage. You can't really do much to accelerate that process, unless you're willing to sacrifice accuracy. And the same is true when you start looking through the answer choices--what are you going to do, skim them to save time? That doesn't make much sense--if you're not reading carefully, you're extremely likely to miss the question, and you'll only save yourself 10-20 seconds in the process, since the bulk of your time is inevitably spent reading the passage, anyway.
So I just don't see the benefit of benchmarking for most people. In an effort to meet your benchmarks, you'll probably hurry through and save a few seconds here or there, but you'll probably miss some relatively easy questions in the process. And on an adaptive test, that's a terrible tradeoff. If you hurry on verbal questions so that you have time to answer question #41, you'll probably screw yourself out of at least one question early in the test. If you miss an easy question early, it will change the trajectory of the entire test, and your score will be disproportionately damaged. If you kick butt on questions #1-38, then questions #39-41 won't matter all that much.
I know that I'm in the minority on this, but I think you're better off answering questions thoroughly, methodically, and consistently throughout the verbal section. If you're a slow reader and you run out of time toward the end, guess on the last few questions--it won't kill your score if you're accurate enough early in the test. I once worked with a guy who practiced verbal like crazy and became fairly accurate, but he was a slow reader, and he couldn't finish more than 35 verbal questions in 75 minutes. On his first attempt at the GMAT, he got a 27 on the verbal section, partly because he was so concerned about timing that he sacrificed his accuracy, and rushed through too many questions. I convinced him to answer the first 35 questions carefully, and then randomly guess on the last six questions. He got a 710 the second time around, with a 40 on the verbal section.
So just focus on accuracy on verbal. Don't dawdle, and try to work on becoming more efficient... but don't race through just so you can say that you got to question #41. On an adaptive test, it isn't worthwhile to make sloppy errors early, just so that you can have a shot at the last few questions on the section.
And if it makes you feel better, almost everybody struggles to stay focused on the GMAT verbal section. All you can really do is practice like crazy, and hopefully you'll learn to snap yourself back into focus when you're starting to stray. Maybe try spending two seconds reminding yourself of how badly you want to go to business school, and that might help you to stay on task. You might also try using retired LSAT questions as part of your verbal practice--the passages are denser, longer, and more irritating than their GMAT counterparts, so if you can learn to battle your way through the LSAT passages, you'll hopefully have an easier time concentrating on the GMAT.
Sorry, this turned into a really long post. I hope this helps!
Helping students kick the GMAT in the nuts since 2002... http://www.gmatninja.com.