If you are under the age of 40, this is every 60 months.
In order to exercise the privileges of a private pilot license, you must hold a third-class medical certificate, which can be obtained through an aeromedical examiner (AME). This should be done early on in training as you will not be able to solo an airplane without your medical certificate. Certificates must also be renewed periodically. If you are under the age of 40, this is every 60 months. While if you are over the age of 40, you must renew your medical cerficate every 24 months.