There is no easy answer to your question. There are hundreds of lenders holding the notes on the short sales. The lenders can be very efficient in their short sale process, using a middle man to assist in their loss-mitigation process, thus resulting in a short sale in as little as 60 days. But, there are many more that are deluged in short sales, understaffed, unorganized and hand off the package internally many times before we see any action.
Some questions I would ask you:
Are you represented by a qualified Buyer Broker? If so, make sure that they are contacting the lien holder at a minimum of once a week.
What process is your short sale at? Has a BPO (appraisal) been completed? Have you done inspections yet? Or, has your seller not even done all of their part by providing updated pay-stubs, bank statements and tax documents. As a buyer, these are the type of things that you need to know.
The average short sale seems to take between 3-5 months, which shows that you are still in that average.
Get your questions answered and then, if things do still seem like they are moving along, hang in there because the outcome of home ownership at a reduced (often 10%) price is the prize!