In short sales, there are no definitive time frames the bank is held to in which they must give you a response/approval. The only time lines you can go off is on the short sale addendum to the Purchase Agreement, there are two areas where you can state how long YOU are willing to wait to get an approval from the bank. The default time is 90 days, but if you aren't willing to wait 3 months your agent can shorten that time period on the offer. The default time for the contract to automatically terminate is 120 days for both parties if no extension is agreed upon, and your deposit would be refunded to you. Extensions can always be signed.
Shortsales can be great bargains, but you have to be patient. I've had some shortsales close in as little as 60 days, but have also seen them go on much, much longer. Review your offer paperwork and look for that shortsale addendum, the dates will be listed on there. You are bound to the contract at least through the date written on there, or you risk losing your deposit money.
In some cases, you will see on the listing in the MLS "Shortsale Approved Price". In this case, the bank has already determined what price they are willing to take, and they've approved the Seller's to shortsale the home. You can close on these deals much quicker, say in 2 weeks if cash, 30 days if financing. All the paperwork has already been sorted, approved, the investor has signed off, so once they receive an offer at that price they can quickly sign off. If you offer less than the approved price, you are back at square one as all the negotiations have to start over with the investor.
If you really like the home, hang in there, because often you can snatch up a deal for way below market value. Also, Florida is the state that comes in for number 2 for longest time period before the bank actually forecloses on the home, averaging about 2-3 years from when the Lis Pendens was filed. You or your agent can look at the public records to see when the lis pendens (if any) was filed to give you an idea of how long its been since they were served.