It goes round and round in beta versions until it's bug free enough for public relase. There's no pre-determined number, it depends on how bug reports go.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.