When I first start running a beta version, I'll update maybe once every two to three weeks… four if I'm being especially lazy. As we get closer to release, I'll update weekly. If there's an issue I've experienced that has been fixed, or something that wasn't affecting me but I want to test the fix for, I'll update sooner.
Then when the release hits, I'll run that for a few months before determining if I want to participate in the next beta. I usually join in for major release betas, not so much for minor ones.