I fly glassfibre aircraft, sailplanes actually - structure lifespan is
a topic of interest when you are at 20 000' above ground level flying
at 200 kph plus and the outside air temperature is below -15 degrees
Celcius.

The technical body that co-ordinates the efforts of a lot of research
and development around sailplanes ( OSTIV ) recently published a rather
reassuring article giving an estimated service lifespan of a glassfibre
sailplane of around half a million flying hours. and that was with a
safety factor of at least 1 order of magnitude.

Although yacht hulls are stressed by rigging and water loads I suspect
that the stresses involved ( in a normal cruising boat ) are somewhat
less than experienced by an aircraft structure. The wings on my 25m
span Nimbus regularly deflect by a metre or so in normal flight. Even
using the same figure of half a million hours one gets 47 odd years of
'use', not just sitting in a berth.

I would say that for all intents and purposes the useable lifespan of a
'well built' glassfibre hull is probably in the order of several
hundred years with only minimal attention to care.

Of course if you insist on building a hull only millimetres thick out
of Nomex, Kevlar, Carbon and other exotics, sail it in across Biscay in
a gale with only one hull of three in the water and driving through 5m
high swells then 'all bets are off' and you may well run into
mechanical limits of the structure.

Ian