The Tor Browser Release Calendar can be found here: https://nc.torproject.net/apps/calendar/p/kdGMbGCWbgPMJR5w
The Tor Browser Release Schedule is influenced by Firefox Release Schedule: https://wiki.mozilla.org/Release_Management/Calendar
We aim to produce stable releases at the same cadence as Mozilla releases ESR updates while alpha release should generally follow one week after. We may also put out extra releases (alpha and/or stable depending) in the event of unplanned security fixes from upstream projects (eg: Firefox, openssl, tor, etc).