Firstly, congratulations on getting a 690. By seeing your location in your profile, I am assuming that you're from Turkey which means that you're not from the over represented Indian or Chinese demographic segment ,so I don't think that a 690 will put you at a disadvantage even at a top 15 . You may wanna get in touch with some students who are studying in your target schools and get a second opinion.
Regarding the time you'll need to boost your Verbal score, there's not any definite answer. You'll need to approach it afresh and it may take sometime and frankly speaking at higher scores you can never be sure what score you may get. So even if you prepare again, there's a good chance that you may improve your Verbal score by just 1-2 points, or it may remain the same or still worse it may come down by 1-2 points.
Are you planning to apply for Fall 2012 session or this fall session only ?