Who needs a First Class certificate
You need a First Class medical certificate when exercising pilot-in-command privileges of an airline transport pilot certificate. It is also required for certain Part 121 second-in-command operations and for crewmembers who have reached age 60 serving in Part 121 operations.
Many pilots who are not yet flying for an airline choose a First Class exam anyway. The certificate steps down: after the First Class window closes, the same certificate continues to cover commercial privileges, and then private privileges, for the longer periods the FAA assigns to those classes.
First Class duration
Measured from the last day of the month your exam took place.
Source: 14 CFR 61.23 (eCFR)
What to bring
Complete FAA Form 8500-8 through FAA MedXPress before your appointment and bring the confirmation number. Bring a government photo ID, the glasses or contact lenses you fly with, and a current list of medications.
Dr. Edmondson strongly recommends reading the FAA's Know Before You Go beforehand, particularly if you have a new or ongoing medical condition.
If you have an existing special issuance, bring the authorization letter and any status reports the FAA requires this cycle. Having those documents in hand is often the difference between leaving with a certificate and leaving with a deferral.