I once worked as an aircraft engineer and changed O-rings on systems that had a very low tolerance for failures of any kind. The systems, hydraulic, oil, fuel...were generally subject to more stress than our housings. Although we would change O-rings according to strict servicing schedules, it was not that frequent (6 months to a year) and only then because we had to dismantle systems for visual inspections. There was a widely-accepted view that if something was working well, you should just leave it alone; there are documented failures as a result of 'over-serving' equipment. And seals need to bed in, after which they are very reliable; the most likely time for a failure is after replacing one. Which is why you should always test the housing minus camera after a service/O-ring changes.
On underwater housings, I keep the seals scrupulously clean, but only ever replace them if they show visible signs of wear or start to harden. Keeping them in a plastic bag and out of sunlight will prolong life. A good test for the larger port and housing O-rings is to very gently stretch them in a few spots around the circumference and look for small cracks appearing at the surface, which indicates excessive wear or a loss of elasticity. I've used housing O-rings for up to 2 years with no problems. And I've spotted potential failures by noticing that loss of elasticity.
In general, keep things clean, visually inspect after each dive trip, keep them bagged up and out of the sun when not in use and replace them only if you see visual signs of wear.