League of Legends bans work accumulative, you start with first one day, then 3 days, one week etc. Maximal ban time is a permanent ban. I believe it comes after a 3 month ban. Please don't fool anyone with saying that your internet crashed you wouldn't get banned for one leave. read more
The Leaver Buster system issues bans between 1 hour and 7 days depending on your Leaver Level. As the number of leaves against your account increases, you will receive harsher penalties that could culminate in the permanent suspension of your League of Legends account. read more