When it comes to team-based games like Overwatch, there will always be times where your match-made squad of strangers doesn't quite ‘gel'. However, even if your team is outclassed, Blizzard doesn't want players to just ditch matches the second things go bad, which is why a penalty has now been introduced for frequent match rage quitters.
Those that frequently quit matches in Overwatch will now face a 75 percent reduction in experience points. If your “games played” percentage falls too far below your “games completed” percentage, you will get a warning in the corner of your screen. If you then continue to quit matches without seeing them through, you will incur a 75% EXP penalty for all future games.
The penalty isn't time based, so if you go on a quitting spree and end up with an XP reduction, then all you need to do is keep playing and see your matches through to the end until you meet Blizzard's threshold for games played VS games completed. However, one thing to keep in mind is disconnections, kicks due to inactivity or crashes will count as you quitting a match and will count towards the penalty.
You can leave a match safely before the “assemble your team” phase is over. You can also leave a match safely after you have seen the ‘Victory' or ‘Defeat' screen. If you leave a match at any point in-between, then it will count against you.
Blizzard's threshold is quite high, so leaving a few games in a row probably won't affect you. Percentages are all based on the last 20 games you have played but Blizzard hasn't stated exactly how many matches out of 20 you would be allowed to leave before facing punishment. You can find Blizzard's full breakdown of this system, HERE.
KitGuru Says: For regular play, Blizzard can be a bit more lenient on those quitting matches before they finish. However, once competitive mode comes out, there may need to be some stricter rules in place.