How exactly does Leave Buster work?

Solution 1:

From the unofficial Leave Buster FAQ (click for more details and information)

How does it work?

Leaverbuster uses a 'level' system to determine how many games you have left. Essentially if you leave games, you will get a warning. If you continue to leave then your account will be suspended. When your suspension expires, if you continue to leave, you will get a longer suspension. This will continue.

How does it determine when to ban me?

The Leaverbuster system has a hidden number that increases and decreases when you AFK/Leave, or when you play a streak of games without leaving/AFKing. This number jumps up much quicker the less games you have played - so if you have only 10 games played 1 leave could get you on leaver level 1. If you have 100 games played, 1 leave might not result in any action - although the hidden number in the background will increase.

Edit: To clarify, there is no explicit answer to your question. Riot wouldn't release that information, it could open the system to exploits. The only way to get concrete answers is to talk to people that have been punished by Leave Buster and collect those statistics

Solution 2:

The FAQ doesn't go into all the details, but says that bans can range from 1 hour to 7 days.

It also states that leaving ranked games is worth double the penalty points of leaving other games.

Anecdote: I believe one of my friends got a 1-hour ban with 4-5 leaves in his first 20ish games on an account, but he straightened up since then and hasn't gotten one since, although he has been forced to leave a couple times.