your application receipt (I797) can give you 180 days of extension. http://www.uscis.gov/archive/archive-news/questions-and-answers
If their post-completion OPT expires while the 17-month extension
application is pending, students who timely filed their STEM extension
applications with USCIS will receive an extension of employment
authorization after their current employment authorization expires, but for
no more than 180 days.
