Road Wheels & Road Wheel Recommendations?

How often should I replace bearings on novatec hubs? I have done about 2300km on them; they are running true and fine but is there sort of like expiry date on bearings?

Replace them when they are fucked, could be anything from a week to a century (that's a hundred years, not 100 miles) depending more on the conditions than the riding time/mileage.