With normal wear an tear, the o-rings on a dive product will remain in great condition for at least 2-3 years. Those who enter salty or mucky conditions more frequently than the average diver will see accelerated wear and tear. If the bezel is ever removed you should thoroughly inspect the o-rings. Remove, clean and grease the o-rings and o-ring grooves, and bezel threading as often as required to ensure a good seal and to give you peace of mind.
Muck diving or bottom shooting typically demands that you remove and clean the o-ring system every time you open the camera housing. Shooting mid-water column usually doesn't pollute the O-rings as much.
You should replace your o-rings if an o-ring is pinched, cut, nicked, or otherwise unserviceable.