I have a GFRP/CFRP glider (EASA CofA) that is two years old. Based on fatigue tests carried out on wing spar sections it has a maximum life of 12,000 hours. This is subject to passing special inspections at 6,000 9,000, 10,000 and 11,000 hours. After that it is scrap.
Some gliders have somewhat shorter airframe lives of only 3,000 hours so it clearly depends on the design of the particular airframe.
My previous glass glider was moulded in 1972 and in its life has had little maintenance to the 'plastic' parts - but a few repairs to metal fittings. However, this is exceptional because it had a very good gel coat. Later gliders used different gel coats that were prone to cracking after a few years (less than ten in some instances) and have had to be re-finished at significant cost (usually in Poland). So again it all depends. Incidentally the 'good' gel coats were alleged to be carcinogenic hence the change - but they were also more difficult to work so it may have been cost-cutting that used inferior gel coats. Some makers are going back to the earlier gel coats that seem to have indefinite life.
My current glider has a polyester finish on top of the gel coat (a factory only option at extra cost) - so hopefully that will avoid the need for it to be re-finished prematurely.