Absent some kind of abuse or extraordinary stress (like falling on top of the bike while it's lying on the ground - stressing the tubes from the side, riding with a friend sitting on the top tube...) a carbon fiber frame should outlast the rider.
Lennard Zinn covered durability of Carbon Forks for Velo News a while back. Here are some quotes from that article in case the link breaks:
For carbon forks in
general, there should not be any
limited life span, as carbon
composites themselves are not subject
to fatigue failures as metals are. So
the fatigue life of a properly made
carbon composite is 'infinite'.
There is no limitation because carbon
has a natural flexibility. It can be
used a hundred years while maintaining
the same stiffness.
The good news is the fatigue life of
carbon fiber is immensely more than
that of metals. While the writer
expresses concern about his carbon
fork lasting as long as a metal
component, there is nothing to worry
about in terms of fatigue life on a
Having ridden since the early 80's I've been through this twice, once with aluminum frames and now with carbon fiber. As high end competition bikes were made of the materials there were questions of durability. Over time, as the frames became widely available, those concerns were shown to be largely baseless.
I believe most frame builders warranty carbon frames for life now. I personally know of at least two riders with carbon fiber bikes that have been ridden in excess of 100,000 miles.
That said, you should periodically inspect the frame for cracks and wear, especially at stress points like around the bottom bracket (just like you would with any frame).