Allie, if that's all you're willing to wait, put in 60 days. If however you can or will wait longer, put that period in there now so you don't have to worry about adjusting it later. The previous posters are correct, it could take much longer than 60 days (I've been involved in one that went almost 8 months before we got approval), or it could be done in 60 days.
As for your earnest money deposit, I would not allow it to be deposited when the seller signs the agreement. I would make sure to have a clause added to the agreement that stated your check is not to be deposited until you get approval from the bank. That way if they take too long, you don't have to worry about getting a check cut to you; you also won't have to have a couple thousand of your dollars in comeone else's account, interest free.