And at first I was annoyed too. But then I figured it out - there are two things mixed up here. Yes, Google really did tighten permissions in the 15th version, that's a fact. But the constant requests often happen not because the system forgets permissions, but because apps request them incorrectly or just work weirdly with the new access logic.
The main thing you need to do is not just hand out permissions to everything left and right, but keep track of who's actually asking for them. Different things helped in different cases: first, I checked for updates in the Play Store - older apps can have compatibility issues with the new system version, they request access incorrectly. Second, here's what actually helped - I went to Settings → Apps → found the ones that kept pestering me with requests, and in their permissions I set the needed one once in "Allow all the time" mode, if the app allows that. Some apps need a cache clear, then they stop asking for permissions again.
If even after granting permissions the system keeps bugging you with the same app - try just uninstalling it, clearing the cache for the "System apps" system application, and reinstalling it. Usually that quiets it down. Just wait a bit after each permission - the system's a little slow updating the status.