There are different types of games however the main LoL game is on Avg 30 - 45 mins. Can be over as early as 15 - 20. Not staged, but your “champ” levels up throughout each game.
Basically it’s a 5 V 5 arena. There are 3 “lanes” which are basically paths from your base to the opposite teams with a river splitting the two and a “jungle” element.
Generally there will be 1 champ from each team duelling in the top and mid lanes, 2 from each team duelling in the bottom lane and a jungler on each team farming through the jungle and roaming to all 3 lanes.
Each champ has 4 basic skills, with a couple of extra “spells”. The game is actually quite easy to pick up and get Into due to the simplicity of the basic play and standard game time however is also quite complex and strategic which is what makes the pros so good.
Each teams base produce “minions” who walk through each lane, you farm the minions whilst attempting to kill the enemy, getting gold from killing minions / enemy and buying items.
Game ends when you destroy the other teams base.
Possibly the worst thing with LoL is a large chunk of the players are quite young which leads to a somewhat extremely immature and toxic environment (obviously older ppl can act this way also).