Killerkoolaid summed it up pretty well: clutch bearings being small and taking a lot of heat, load and RPMs should be checked after every race, and replaced as soon as they feel notchy or gritty (usually every 2-3 race weekends.)
It's recommended to tear-down your car and check bearings (as well as other parts) every 2 race weekends. Some who are more maintenance-fanatic probably do it after every race weekend, which is also OK. In short, the goal is to try to catch a potential failure before it happens.
Bare in mind that variables like driving style, your propensity to crash, the track layout (big jumps!) you run on and even the type of surface will all influence how long your parts/bearings will last.