OTA updates usually follow this rhythm:
devel: daily builds
rc: weekly if no critical issue exists in the devel channel
stable: every six through eight weeks, if no critical issue exists in the rc channel
This is not a definitive cycle. Stable releases are ready when they’re ready and should not introduce new bugs or ship very incomplete features. Since the OTA update can not be released before all critical issues are closed, the ETA might have to be moved by making an educated guess for when all the issues can be handled. If there are too many issues added to a milestone, they are either removed or added to the next milestone.