After leaving a few games, you will be placed into a low priority queue; as long as you can complete 5 matchmade games without leaving, you will be returned to the normal queue. However, if you continue to leave games, you will remain in the low priority queue.

How long does low priority queue last?

Generally, summoners will be placed in the low priority queue for five games and will have to wait for five minutes before finding a game. If a player continuously leaves the low priority queue, however, timers increase from five minutes to 15 minutes per game.

Does low priority queue go away on its own?

Yes it goes away after 5 matches as long as you don't leave games anymore.