The listing agent can call the GMAC negotiator of this short sale to get an estimated time line of when they will send out the short sale approval letter. GMAC should let the listing agent know if they can move forward with the information they have already gathered and not start over entirely. Have your agent speak with the listing agent & discuss timelines. Usually the BPO (broker price opinion - appraisal of home value) is good for a certain period of time after it has been completed, so several of the steps of the short sale approval process do not need to be repeated. Additionally, the seller's hardship package, etc has already been submitted to the bank and is approved at that offered price, so that's a HUGE step in the right direction in regards to whether or not the bank will approve the short sale.
That being said, it may be a couple weeks just to push the paper around at GMAC, submit the new offer, etc, and prepare the short sale letter for the seller to approve and escrow to start.
I had the same situation where the previous buyer back out of the offer, and we snuck in with an offer at approved short sale price and got a response within 2 weeks or so, and closed 30 days later. Either way, you should hopefully get a response back faster than going in blind on a short sale property that did not have any offers yet.
Good luck with escrow!! Be sure to do all your investigations and inspections!!