That would depend on what your contract states. Your Realtor should have done the Short Sale Addendum (C.A.R. Form SSA) when you submitted your offer to the seller. Look at the SSA and see if your Realtor indicated contingencies will start once you have written approval from the lender(s). Hopefully that is the case. And, if so, the contingency period would start on the date the lender(s) gave written approval of the short sale.
The 17 days depends on your offer you made in the contract If you put 17 days then that is what you have or made no change to this as it defaults to the 17 days. If you put a different amount of time say 14 days then that is what you have
Good luck on your offer
Ingrid Ski Realtor
Real Estate Broker
Unless anything is written in your contract to the contrary, your 17 day contingency period commences the day after you receive written short sale approval.