I have had short sales that were smooth transactions and closed in 2 months and I have had nightmare transactions where the banks greatly increased the frustration and difficulty levels.
There are extra steps involved in short sales.
First, the seller has to accept your offer same as a normal sale.
Then they need to prove to the bank that their financial situation is bad enough they can't pay the mortgage. This is the step that usually takes the most time since banks can ask for a lot of documents from the seller and sometimes take a while to review them and then may ask for more documents and reviews, etc.
If you get through these steps, the bank will have the home appraised and decide if your offer is acceptable to them. A lot of deals fall apart when the bank comes back with a higher price. It's important for the selling Realtor to have done a good job pricing the home to avoid appraisals coming back too high.
After the seller review and appraisal processes have been successfully completed, the bank may have more paperwork for the buyer. Once this paperwork is returned and approved, the sale can finally proceed and close usually within 30-45 days. The bank limits the time their approval is good for so the sale NEEDS to close within that time period or the buyer's agent will need to ask the bank for an extension which may or may not be given. Remember, these homes may be in the foreclosure process so the bank may decide to foreclose if the short sale does not close on time.
2nd mortgages make these transactions lengthier and more difficult. It's hard to get one bank to agree to take less money than they are owed and harder to get two to do so, especially since the 2nd bank usually won't get much from the sale.
Most banks require buyers sign an arms length transaction document so the buyer cannot sell or rent the home back to the original seller.
In general, I tell my customers a short sale will usually take between 2-6 months, in rare cases longer. If they can wait patiently, they may be able to save a little money off the price of a comparable non-short sale home. If they need a home soon or don't have the patience to deal with a bank's processes, a short sale is not the home they should be targeting.