Most likely reopening: Sep 1
Given current speed, but also pretty sound predictions ([URL]https://www.n-tv.de/politik/Durchimpfung-bis-Ende-Juli-ist-moeglich-article22526205.html[/URL]), a realistic scenario is that by Aug the majority of Germans are fully vaccinated.
Consequentially Sep would be a good timeframe for reopening. Optimists like me, might even hope for July / Aug. But I guess folks will only be granted access if immunized. I have just got my first shot last week.
Pessimists dream up all kinds of other possibilities (new sars-cov2-variants, etc), and then everything is of course unclear.