Android P is expected to be released by Google sometime in Q3 2018 (October - December). The 6.1 should get a release sometime not too long after that. It's too early for anyone (even Google or HMD) to give a more precise expectation. The Android P API isn't even finalised at present, so there are many variables that could push the date out. The current developer preview is incomplete and unstable; intended only for developers (or equivalent), and not suitable for general use.