Questions about Key drop rate reset date

So the more keys you have gotten, the less chance you get keys fragments in future games. But this resets every month. (confirmed)

But does this drop rate % reset at the start of a new month (April 1), or after 31 days have past since obtaining your first key (2 weeks ago or so?)

Just curious to know, because by now I'm on my last chest and i'm very near the 0% drop chance for key fragments when solo queue'ing in ranked.