ATP
Actually, it like the FE depends. Normally they are valid for 24 months. However, if during that 24 month period, if you are employed as a pilot by a Part 121 air carrier or a Part 135 commuter air carrier, it is valid indiffinately. The only requirement is that you when you use that written, you work for a Part 121 air carrier or a Part 135 commuter air carrier and have gone through their PIC training program.