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Everyone gets SOME degree of leeway in leaving to account for legitimate causes, such as someone in the home needing to go to the hospital, a sudden power outage, etc.
But these sort of things are normally RARE. If someone leaves a lot, chances are they're just making up excuses. Also, "Brb gonna get a beer" is illigitimate a reason on two fronts:
One should not be inebriated while playing. It's hilarious to you, but NOT to your teammates who will be far more likely to lose.
You ought to have any refreshments/needs done BEFORE the match. If something's necessary that you couldn't live without for 30-50 minutes, then grab it quick while your champion's dead or something.
TOO MANY people get into matches without being responsible: the game DOES warn that a match in Summoner's Rift typically goes for 30-45 minutes, and I'd make sure to block off AT LEAST an hour to play a match.
So if anyone leaves, I recommend reporting them. If it was legit, then it was rare, and they won't be punished for it.