Typically, the closing date is written into the offer. It's one of the things they can come back with to negotiate - particularly if they are moving out of their home and into a new one. Ours was 30 days, but I cannot say for sure that it's the norm.
The biggest hang-up tends to be the bank, though. As long as your finances are in order (make sure you're pre-approved) then hopefully you won't have any snafus. Our bank was down to the wire with the final approval.