About how many games does it take to get mastery 5 on a champion?

If I remember correctly, Mastery 5 takes ~21000 points to reach. Wins give you ~1100 points while losses give you about ~200.

Assuming a 50% win rate you should get Mastery 5 in 33 games.

That's assuming you have a 50% win rate.

In my case since I have a 30% win rate it would take me 45 games xD

It depends. You get mastery points for winning and losing, but the points awarded for losing are significantly less.

I think it changes with how long the game is

20-30 games depending on wins/loss and grades per game

around 21~ wins

roughly 20. with prob 15 wins with them in that time. I did it with morg in exactly 20 matches