Chris, Short sales are all different depending on the stage they are at. As soon a homeonwer defaults they have a specified time by law to "cure" the debt and pay the bank to "catch up" thier payments. If the bank has already filed the "notice of default", which is public information then the homeonwer has a scheduled amount of time to "cure the debt". This shceduled period of time is 90 days plus another 21 (111) days. However, if the the borrower files for bankruptcy or files a lawsuit then this can also add time.
So you can see by my ver simplified explanation that there is no magic number or time period....the are all different.
If you are the purchaser of a short sale try to find out if the one you have the offer on has already been served with the notice of default.
As far as the bank responding to your offer, it depends on the bank. Most of them are very understaffed. Remember that banks were never meant to be owners of real estate, so they are un prepared for this huge inventory that they now have to unload. Banks primary role in real estate was to furnish loans that borrowers could pay back.. Then there is the legal aspect, they all have legal departmets that have to look at each offer and this takes time. It also takes time if there is more than one trust deed holder.
If it is a home that you are really hoping to buy, you may want to hang in there and wait...they will get their act together and sell it eventually. Try to have your agent investigate if the N.O.D. has been filed and how many banks or trust deed holders there are. Good Luck!