Short answer: You have 180 days (6 months) from the denial notice to file an internal appeal, and 4 months from a final denial to request an external review. Plans must decide an internal appeal within 30 days for care you have not received yet, or 60 days for care already received; standard external reviews are decided within 45 days.
The clock starts when you receive the denial. For an internal appeal, you must file your internal appeal within 180 days (6 months) of receiving notice that your claim was denied. Insurers also face deadlines to issue the denial in the first place: within 15 days for a prior-authorization request, 30 days for services already received, and 72 hours for urgent care.
Once you file, the insurer must complete a standard internal appeal within 30 days for a service you have not gotten yet, or 60 days for one you already received. If the denial stands, you have four months after the final determination to request an external review, which a standard reviewer must decide no later than 45 days after the request.