Nylon does not "fatigue" the way a metal bolt will (repeated mechanical stress on metal bolts causes hardening and ductile stretching of the metal). Nylon is not, however, without its faults. Nylon is not a UV stable plastic, which means that overtime, exposure to sunlight will cause the nylon to become brittle (just look at any nylon rope left out in the sun for long periods).

The reason for using nylon bolts in most airplane applications is to get the bolt to fail before the wood does, protecting more of the structure in a crash. For them to operate correctly, they must be prestressed when they are installed. Typically tightened hand tight, and then turned 1/4 to 1/2 turn more to stretch and preload the bolt. Being plastic, it will stretch over time in this condition. Since they're cheap, replacing them occasionally (annually?) is just good insurance.

Brad