Students with a STEM degree and currently in the 12-month OPT period can apply for STEM OPT as early as 90 days before the expiration of 12 month initial OPT period.
USCIS must receive your STEM OPT extension application before your current 12-month OPT EAD card expiration date.
Some schools require up to 2 weeks to process your I-983 form and issue new I-20.
Then, also consider the time to mail the application to USCIS via FedEx.
WARNING – USCIS will reject the STEM OPT application received after the expiry of 12 Months Initial OPT. To be on the safer side, make sure it receives atleast 7 days before the expiry of 12 month OPT Card.