M7 on all champs

If you are only playing with that goal? Like, spamming normals just to get the champs mp non stop? I'd say if you are good about 20-25 hours of game time per champ from 0 pts not accounting for time to gather BE, Im assuming you only need to get 0-22k and then 5 S, and that you will get those 5 S really fast like 10 games max for those

Well, to get mastery level 5 on a champion you need 21,600 mastery points. In a well played game. (6/10) You gain about 500~ish mastery points. You can gain up to 10% more if you are in a full premade. So, about 550 points per game.

I will use a 30 minute game as an average. That would be 21,600/550. That equals about 45 games. 45 games times 30 minutes a game is 22.5 hours. 22.5 multiplied by 144. (Up to Yummi) would be a grand total of 3240 hours playing the game. Or about 135 days straight. That's only for level 5.

Now, let's assume that you get at least an S- every 5 games on every champion. That adds 10 games on every champion for level 6. 1440 games multiplied by 30 min games equals another 770 hours bringing the total to 4010 hours total.

However, we're not done yet. We still need level 7. Let's take another guess and say that every 10 games you can get an S rank on each champion. That's an additional 2880 games and 1440 hours of playtime. This would bring the grand total number of hours, based on nothing but a random dudes generalized math to a total of 3240+770+1440= 5450 hours!

That's 228 days straight! No sleeping, no leaving your computer, no load times, exactly 30 minute games, no server downtime, no patches. So yeah, it would take an obscene amount of time to do. Bear in mind that this estimate makes the ridiculous assumption that you are very skilled on all 144 champions and assumes that no new champions are ever added. But hey, if you want to do this go right ahead! It's a huge commitment, but if you have the willpower, it can definitely be done!

TL;DR You would have to play for 2/3 of a year straight with four friends playing very good on every champion for all Mastery 7.