Hello Arti-
I'm sorry to hear about your knees. That is quite unfortunate that both your knees have torn meniscuses.. meniscusi? Not sure what the plural would be for that, but either way, it doesn't sound fun.
Though I dont have any personal experience with it, I would suggest getting both done at the same time. Only because.. well, would you really want to undergo surgery twice? I'm all about getting it over and done with as quickly as possible.. Keep us posted on your progress once you decide what to do though! It'll probably help you during your PT to come here and vent about how much everything sucks or something.. 
As for how long it's going to take, I think it probably depends on how your surgery goes and how badly your knees are injured. I would say ask your orthpedic surgeon for specific details.. Or maybe you could post a comment on this user's blog - he underwent "scope surgery" for a torn meniscus a few months ago...
Another useful resource might be Knee1's Preparing for Surgery section... Good luck!