It is valid forever, but in order to act as a required crew member you need a flight review in the previous 24 calendar months. A flight review can be conducted by an examiner, inspector, or CFI.
In order to carry passengers, you need to perform 3 landings in category, class, and type (if requiring a type rating) in the previous 90 days.
Refer to FAR 61.57.
Last edited by zondaracer; 16th Sep 2013 at 11:42.