It depends on how well you look after it. I've seen RIBs several years old that look better than the day they left the factory, and I've seen RIBs looking wrecked after a single season being left outside uncovered.
On the other hand, even the most neglected RIB will keep working and it is amazing how a bit of effort can clean up an old boat.
Avon offers a "10 year transferable, no-qibble written fabric guarantee and 5 year seam warranty" and have a 20 year old searider that they like to show off.
Zodiac offer a 5 year warranty.
The tubes will be the first area to wear out, but they can be replaced reasonably inexpensively (compared with buying a new boat) which makes the boat like new. Henshaws specialise in this, and seem to be doing a good trade replacing damaged PU/PVC tubes with hypalon.
To get back to your question, as a rough estimate I would say that a RIB that is lightly used and looked after should easily give ten years of use.