raspbian – Debian Bullseye Compatible Raspberry Pi OS

No one can answer your question – it is unanswerable, just as the question, “When will the universe collapse on itself?” You’ve asked for an opinion, not an answer. That said, it seems likely that the bullseye release will happen before the universe collapses.

With respect to the “in-place upgrade” you’ve referred to, it may be a good learning experience, but unlikely to turn out as well as you’d wish. Raspberry Pi OS has never provided a sanctioned/guaranteed “in-place upgrade”, and it seems highly unlikely they will start now.

If I were determined to try a bullseye upgrade in advance of the official release, as an “in-place upgrade”, I would read RonR’s post, and try his upgrade script. Why? He has something quite rare: credibility.