USCIS will take a whole lot of time. My parents extension application is still pending. I filed on Dec. 1 and after 2 months it was transferred from Vermont to California and it is still pending there. good thing is that they can stay until heard from USCIS even after the I-94 expiry. USCIS will give you 30 days notice to leave the country if the extension is rejected.
p.s. it is wise to file 6-8 weeks before I-94 expires.