It's odd that we measure most things aeronautical in time, not distance or even speed. The relationship of time to aviation regulation is fundamental and is the basis for nearly all rules that govern pilots and the operation of aircraft. Log the time and you're legal to fly, but don't forget to check the calendar as well as the flight hour meter.
Perhaps the first time restriction a student pilot has to face is the limit on solo privileges. When your instructor endorses your student certificate to allow you to fly the airplane on your own, the solo endorsement is good for only 90 days. After 90 days you can't continue to fly solo until your instructor has renewed your privilege by again endorsing your certificate [FAR 61.87 (n)].
The 90-day solo endorsement is also good for solo cross-country flights, provided you've been signed off by your instructor to conduct cross-country flights. But in addition to the 90-day endorsement, you'll need to get an instructor to review your cross-country planning and endorse your logbook for each specific cross-country flight. The instructor who reviews your flight planning does not have to be your regular instructor.
As you progress with your flight training, you might run into another time restriction. Your student pilot certificate expires 24 calendar months from the month in which it was issued [61.19 (b)]. So if you've taken your time soaking up things aeronautical and two years have passed, then you'll have to get a new student pilot certificate.
You'll notice the expiration was based on "calendar" months. When the FAA uses calendar months it means the period from the beginning of a month to the end of that same month after the time has expired. So 24 calendar months means that if you got your student pilot certificate on March 1, 2003, it's good until March 31, 2005.
Once you've successfully passed the written/computer knowledge exam with a score of 70 or better, you're faced with another time limit. The results of the exam are good for only two years. So if you haven't concluded the aeronautical experience requirements in time to schedule and complete the practical flight test within the 24-calendar-month period, you'll have to take the knowledge test again [61.39 (a) (1)]. Pass the test on or near the first of a month and you get almost 25 months to complete training and take the flight test.
Okay, so you've completed the required aeronautical experience, passed your written and scheduled your practical flight test within two years of having passed the written. But you're a bit nervous and things don't go as well as you'd like during the flight test. The examiner isn't happy either and tells you that you've failed the practical and hands you a pink slip detailing where you went wrong.
