Third class medicals have a validity period of 5 years that turns to 2 when the pilot turns over 40. The same goes for second class medicals too. However, after the pilot turns 40, his second class medicals turns to third class after 2 years. First-class medicals are valid for 6 months to 1 year depending on the pilot’s age.
If you suffer from any medical condition that will disqualify you, then you should search for the same beforehand itself. This will help you know what to expect when you show up for the exam.