My personal experience is 8 days to 21 weeks.

How's that for narrowing it down?

There is no way to answer this question. It depends on the lender, the loss mitigation officer, the agents involved, the type of loan, whether or not there is a second, the payoff amount, a healthy does of luck, and apparently the phase of the moon. And other factors.

Just like Yolanda responded- it really depends on the bank you are dealing with and the agent behind the short sale efforts. The offer must be solid with all required forms and proof of funds or a Letter of Pre-Approval the buyers lender(called LSR here in Phoenix).
You need to know that it will take time and bring a big dose of Patience because as the buyer and buyers agent you have no control and at the mercy of the lender or their negotiator to respond.
Put your highest and best offer at the beginning and have a better chance of getting your offer accepted especially if other offers are in line or come is with yours.
Look luck and make sure you find yourself an agent who has a track record of working with short sales and also being a strong Buyers Agent. Look at designations from your realtor for that tells you that they have worked hard and educated themselves to better serve their clients or customers

Cecil Duarte -Broker/Owner
Serving Valleywide Realty

It really depends on which bank you are dealing with. It could go from one week to months. I use a title company to do the calls and get the offer through to the right person. It really speeds things up most of the times, since that's all their shortsale department does.
Good luck
Yolanda Varela http://www.yvproperties.com