I was wondering how often key fragments would drop from every match, because it's been a long time since I've gotten a key fragment.
I got 3 chests ready to open and I currently have 2 key fragments, I just need one more...... 
I've played less than 10 games this week and I have gotten two key fragments.
From the official FAQ:
"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."
And of corse you need to win games and the droprate is higher if you play with premates
