Originally Posted by
Branko D
It's illusory to talk about bike life span in terms of years. Bike frame fatigue which happens inevitably with all materials is a product of mileage, riding style, rider weight and power (out of the saddle grinding subjects the frame to much higher forces than climbing in the saddle, even higher than hitting road bumps), amount and severity of manufacturing defects (which happen, even in metals; just ask all the people who had a Ti frame break at the welds) and any impacts which may have created dents / cracks / delaminations / etc which may have weakened the material.
You can go down the rabbit hole of examining articles about bike and fork fatigue testing and so on, but it's really hard to translate how many cycles of what force to miles on the road, especially as there's a huge difference in terms of forces between eating up miles spinning on the flat on a smooth road, and a ride in the mountains over crap roads.
Not to mention it will depend largely on the individual bike model too. It's really a non-starter of a question to answer with any meaningful single number. Depending on the specific bike model and usage, expected lifespan could be anything from a few years to many decades. There's nothing inherent to carbon fibre that would limit any frame made out of it to a specific limited lifespan. It's not going to melt, dissolve or fall to pieces merely as a result of the passage of time.