How long do Carbon frames last for?

Depends on the frame, same as
Depends on the frame, same as alu or steel really, some manufacturers give a life time warranty, some don’t (obviously the warranty would be void if you were buying second hand anyway), but I’d take it as an indicator of longetivity.

Those with the former you’d reasonably expect to last a long time. I did once talk to a manufacturer of top end bikes and he based the life expectancy of his frames on five years of a pro riding it and then a bit more. Most of the rest of us would never manage anything like that amount of miles before we wanted another bike.

As for buying second hand carbon I’d say the advice is petty much the same as for buying any other second hand bike, look for cracks, blemishes and signs of wear and tear around the stress points on the frame – and if you find any avoid it.

Haven’t heard of their being any particular problems with lugged carbon frames. I might be a bit circumspect about buying a five year old super-lightweight frame, but I’d probably feel the same about a steel or aluminium frame. You won’t go wrong with anything from the major brands, Giant and Trek were in to carbon very early so they know what they are doing and to be honest I can’t think of any brands to avoid. Again, if it was one I’d never heard of I’d probably want to do a bit of homework before I handed over any cash.