My concern would be more with getting back into the country than leaving. If you haven't heard back do you even know the application was received? Things do get lost in the mail or some clerk at the USCIS could have misplaced it. You never know.
My advice would be to call the USCIS and get a confirmation of your application. Or better yet if you can, go to a USCIS office today (since you're leaving tomorrow) and get something in writing that you sent in the application.