A new stable release takes place every couple of months (23.05.0, 23.05.2, 23.05.3 etc) - sometimes new devices get added in this cycle.
A new major release takes considerably more time due to porting targets to a new kernel - see https://github.com/openwrt/openwrt/issues/15192
After porting has finished another few months are needed to iron out bugs - I predict that next major release is in H1/2025
Sidenote: developers are also working on cool new features like
- APK The future: opkg vs apk
- APUP https://github.com/openwrt/openwrt/pull/15442
- UFP https://git.openwrt.org/?p=project/ufp.git;a=shortlog
- OWUT Owut: OpenWrt Upgrade Tool
- QAM-256 on 2.4Ghz https://github.com/openwrt/openwrt/pull/12904
- LLDP https://github.com/openwrt/luci/pull/6456
(some of these will become/are available in 23.x as well)