Quality fiberglass can last almost indefinitely if not left bare to the elements. I have a 66 Vette and it's very sound. That being said - fiberglass does react long term to stress, heat, sunlight in that it can reflect stresses in the surface and get minor waves in it's surface. The better fabricators avoid bonding interior panels directly to exterior panels due to the eventual print through that can happen over years and decades due to heat of bonding and stresses the interior panels convey to the exterior. If an interior panel needs to be bonded to the body it's best to do it around the edges of the hood or trunk openings, at the rocker line, and at the edges of the cockpit - so it can't show up in a large flat or gentle contour surface. It's also a good idea to protect the underside of the body since raw fiberglass can pick up contaminants like brake fluid and AC compressor
oil can and will work through to the surface and lift paint.