We were involved in a short sale as well and they are a mess. The bank can take as long as they want to respond to your offer. We made the move of putting a time limit on our offer. It got them to accept in a more timely manner.
Why do they take so long? (I'm sure those "in the business" will disagree but I have talked to several people and this is what banks are doing.
1. The longer they can delay the acceptance of an offer the longer the property stays on the market and the greater the chance for another or better offer to come along. That leads to a bidding war which drives the price up.
2. Short sales aren't actually bank owned yet, so their is no real incentive to sell them. Banks would much rather get foreclosed homes off their books. They delay short sales to the point that no one wants to go near them so they are basically taking these houses out of the market. In essence, this removes these houses from the inventory which allows the banks to clear their toxic assets (foreclosures) from their books at a better price. They are putting a false bottom in the market.

I would suggest giving the bank a drop dead date for offer acceptance.

Please remember that short sales can take 90 days for approval. Loss Mitigation Departments at mortgage companies are so overloaded that it takes a great deal of time to get anothing done. The best thing is to have a selling agent who has great communication skills with the bank. The worst thing is to have a selling agent who has terrible communication skills with the bank. You'll probably find out soon which you are dealing with. If you're agent has added an addendum in your offer that the mortgage company has to give approval in a certain time frame, you've at least got some kind of idea of the time you are looking at. If not, you might be in for a long ride before you have any communication. Have your agent stay on the selling agent. That's the only thing you can do.

Short sales can try the patience of everyone involved because (1) the seller needs and is anxious to sell, (2) you have a willing buyer, and (3) the bank(s) have the say-so and control the speed (or lack thereof) for the process. If there is only one lender involved, it is more likely you can work the deal out. If there are two lenders involved, it is more complicated to work out (and can take even longer) because the mortgage holder in the second position is at the mercy of the first mortgage holder. Sometimes that means a "stand off" and the deal stalls out. The "bank representatives" are not the ones making the decision, but the offer has to go before a committee. One bank, one committee. Two banks, two committees. With all the foreclosures happening.... well, you get the picture. The company I work for has a Foreclosure Department so this is an area that I am very familiar with. Your agent should check with the listing company / agent (the company / agent whose sign is in the yard) to determine the status -- that is the only line of communication in a short sale. Sometimes the communication can involve leaving messages back and forth due to the sheer volume of calls into those departments at the bank(s). With all that being said, I know it sounds bleak, but I wanted to give you a realistic idea about what can cause the delays. Let me also encourage you to have confidence in your agent, and be as patient as you can be, because short sales do usually take longer than a normal transaction. Best wishes to you and I hope it all works out for the best for all parties involved.

The short sale is a process to go through. In my experience, if you have an account representative to talk to at the bank, you get more accomplished. You cannot talk to the bank because you won't have permission to speak about the loan in default. The listing agent would be able to talk to the bank if they have the sellers permission in writing. Your agent will have to keep in contact with the listing agent, but it can be a very slow process.