AFK is handled by LeaverBuster, not IFS. LeaverBuster is the strictest punishment system. Here's how LeaverBuster works:
Are they idle or AFK for more than 3 minutes? If yes, pop up a warning in-game.
Are they idle or AFK for more than 5 minutes? If yes, give them a strike.
Do they have any strikes this season? If no, give them a warning in-client and force them to type "I Agree."
Do they have exactly one strike this season? If yes, give them a 5-minute timer added on top of five queues.
After 5, the timer goes to 10. After 10, the timer goes to 20. Then the timer never goes down until next season when the strikes are reset.
Note that, to avoid LeaverBuster, you must kill units or interact with champions. You can't walk in circles in base. Well, you can, but you'll still get a strike.
Riot stopped handing out bans for AFK years ago.