You can insist on seeing evidence of the bank's receipt of the offer but in some instances that's difficult to prove.
In my experience, banks can wait as long as they like and often ignore any acceptance date/time deadlines.
Banks are overwhelmed with short sale offers all over the country right now and in many cases there's no particular personal motivation to approve paperwork. We might like to think the opposite, but the banks have been showing us that contrary to what intuition tends to tell us, some banks don't seem to be interested in getting paperwork done quickly.
There are exceptions and I've seen some responses in reasonable time (a week or so...).