I work short sales and I can tell you that a Short Sale that an agent is advertising as "approved" can be quicker if, you come in at the price that was approved in a short time (within 30 days typically) from the original approval. Most often a property will have a 1st buyer that gets the process started and then a 2nd or even 3rd buyer that comes in and buys the home in a lesser time than the first buyer waited before walking.
Some lenders are approving quickly when the seller has had a failed modification. I had one that Bank of America approved and I got closed in 3 months from the contract because the sellers had been working with their lender for a long time trting to modify and were denied. Bank of America is trpically taking many months to even review files.
If you work with a Realtor to make your purchase, make sure that your agent asks the Listing agent for as much information possible. There are no gurantees with short sales. If you have a timeframe, you should look at Bank Owned or Traditional Resales. An educated motivated seller will price closely to short sales and you can get a great deal without the wait. Best of luck to you.