Coming from a car racing background...there is a reason for belts to be replaced at regular intervals. Dirt, abrasion, chemicals, and UV especially degrade the webbing over time and weaken the belts. Belts have to be replaced/rewebbed every so often (2 or 5 years I believe) depending on the certification they have.
That said, belts in a race car have to withstand massive decelerations (50 g impacts I think? Don't quote me on that.) to be considered acceptable to do their job and in a race car it's basically a foregone conclusion that this type of event will happen sooner or later. Aircraft belts shouldn't (hopefully) have to withstand an impact like that, they're generally for holding you in place in the plane so the requirements shouldn't be quite as stringent. As morbid as it is, if you have a 50g impact in an airplane I don't think the belts would be the weak point in the system.
So how often should you replace your belts? Well this is experimental right...so what do you feel comfortable with? Realistically, the answer probably depends on a lot of different factors. The biggest is probably how much UV exposure they get.
