I had my F-1 visa interview on Oct 3rd 2012 in Dubai consulate general, VO told me my visa is approved but subject to administrative processing, more than 6 months passed since then and I'm still waiting. I've called consulate and emailed immigration office in US and also consulate, they all told me I have to wait and there is nothing they can do. I know waiting is all I can do but how long? Is there any limits?
Also when my visa is approved my semester is finished, can I apply for a new semester in the same college and go to US with the same visa? Or I should apply for a new visa? How long do I have time to enter US when I got my visa?

I would appreciate any information on this specially from insiders.
Thanks a lot.
Mike