There are a lot of factors that have to be taken into consideration. If the seller has filed for bankruptcy, the Bankruptcy Court will have to sign off and give permission for the home to be sold. That can hold things up for quite some time. Liens are another nightmare that can cause a host of delays.
The speed of the short sale can be expedited if the listing agent submits all the requisite documentation at the time they submit the offer to the bank. This will fast forward things as it eliminates the lender needing "one more thing" before they can fully review the file.
I've seen short sales take as little as two months. Others, six months and still others, I don't know if a decision will ever be made....Just remember lenders like communication. Keep them informed, be respectful and if you don't know what you are talking about, be honest! Nine out of ten times you will get someone on the file who wants to help, so work as a team.