I think it's a "when it's ready" type of a schedule. I imagine it will be part of a 24.x release.
You can try it out if you want to see the sausage making: https://github.com/pop-os/cosmic-epoch
One of the devs posted a response to this question in this thread, since I asked the same question:
https://sh.itjust.works/post/69204
If you are technical, you can start playing with it now but no estimate when they'll have an ISO ready.