Once fibreglass has been laminated I'm pretty sure it will last indefinately, subject to it not being over stressed (or driven into a tyre wall like my nose cone
)
Fibre glass matting is, I think, "held together" by some form of emulsion, and when i bought some (for a new nose cone!!) the chap at ScottBader, who was very helpful, said to keep it dry to stop it falling apart.