To answer your question, you may not have to vacate until the sheriff knocks on your door next year. Some people later work something out with the bank and by the time that happens there is water damage in the house, mold and it's trashed. You have a responsibility to minimize the damage.
Sorry that you are going through this like so many others in these difficult economic times.
If you havn't tried to sell it yet, the bank may have other options for you so that your credit doesn't get messed up! Good Luck, the process is stressful and not very much fun!