Three months is the official extension but unofficially they still work for me much longer. I just tested tests that expired last March and April test and they work fine.
You can't use the proctored version past the extended expiration. So mine expired in early December, tried using a few days thereafter and the proctor said I need to order new ones and ended the session.
You can (and I did) still use it for self testing at home but these are more expensive than the regular non proctored at home ones. However obviously better than just throwing them out.