Any OPT STEM Extensions that were submitted before May 10, 2016 will be valid for 17 months. To receive the additional 7 months of authorization, you will be required to complete a second application to USCIS.

If you have submitted your application to DSO last week, there is a chance that he/she has not posted your application to USCIS yet (considering my experience with the DSO advisers, it's like a government job for some).

But you have to wait until May 10, 2016 to apply for your 24 month extension.