What i want is that

https://support.riotgames.com/hc/en-us/articles/207884233-Hextech-Crafting-Guide

FEELS LIKE I'M NOT GETTING MANY KEY FRAGMENTS ANYMORE. WHAT GIVES? The drop rate for key fragments is a curve, not a line. With each fragment you earn, the chance to earn your next one goes down slightly. This number resets roughly every thirty days after you earn your first key fragment.

We wanted to give players who didn't play as frequently a good drop rate for keys, without making it feel like farming was mandatory if you wanted keys.