EAD extension not to be filed prior to 120 days before expiration. 

Green Cards > Adjustment Of Status > EAD extension not to be filed prior to 120 days before expiration. 

Employment Authorization

U.S. employers must check to make sure all employees, regardless of citizenship or national origin, are allowed to work in the United States. If you are not a citizen or a lawful permanent resident, you may need to apply for an Employment Authorization Document (EAD) to prove you may work in the United States.

USCIS issues Employment Authorization Documents (EAD) in the following categories:

• EAD: This document proves you are allowed to work in the United States.
• Renewal EAD: You should apply for a renewal EAD 120 days before your original EAD expires.
• Replacement EAD: This document replaces a lost, stolen, or mutilated EAD. A replacement EAD also replaces an EAD that was issued with incorrect information, such as a misspelled name.