From the Leaverbuster FAQ:
HOW LONG WILL I REMAIN IN THE LOW PRIORITY QUEUE?
You will be placed in the low priority queue for 5 games. If you continuously leave, the low priority queue timers increase from 5 minutes per game to 10 minutes per game and then capping out at 20 minutes per game.
IT'S NOT MY FAULT. MY INTERNET/YOUR SERVERS DISCONNECTED ME. I HAD A REAL-LIFE EMERGENCY. I HAD TO GET OFF THE COMPUTER. THE GAME CRASHED. UNBAN ME!
We understand that players may be forced to leave from time to time so the system has been designed to keep these situations in mind. 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.
If you've done all this and still find yourself in LPQ then try filing a Support Ticket and see if they can help you out there.