Yes we do know:
I think that is pretty much the case. The two OSs are not so different that it takes double the time to maintain them. Fairphone fixes bugs and integrates security fixes. As soon as they get approved by Google the new FP OS is released and the process to integrate them into FP Open begins.
That’s not an option if they want to keep Googles official support. Without it we can say goodbye to Android 6+ and to monthly security updates.