If you are referring to a short sale situation, the answer will vary depending on the bank. Not the answer you were looking for to give me "Best Answer" status? I didn't think so. Never-the-less, that is the case. Since you asked how long it "usually" takes, I would say 45 days is generally accepted as a good number for how long it takes them to sign everything and close the escrow.
Your agent, as well as the listing agent, will have a dramatic affect on that number. Make sure your agent (assuming s/he is different than the listing agent) gets their name put on the list of people allowed to discuss the loan situation with the bank. Banks require that sellers authorize who is allowed to speak with them concerning their loan. The seller can authorize one or both agents. Once your agent is authorized to speak with the bank, have them call minimally twice a week to check the status of the paperwork and to see if there are any items they are missing. If you do as much of the bank's work for them and continue to follow up with them, you can get an escrow to close in 30 days or less. I know from personal experience.
I hope this properly answered your question, Sally.