Can you be banned for AFKing games from time to time, like maybe once a week?

once a week is pretty often. Back when I was playing I only disconnect like maybe once every few months.

Yes you can get permabanned for afk and leaving too frequently. I don't know how many times triggers the ban but I would suggest you do not do it frequently. Try to block out time so that you can play league uninterrupted because you are playing a team game and it's not fair for your team to have to go 4v5

Keep in mind that you ruin the game for 9 other people. Going afk once a week, which is really often, is really selfish dont you think?

You can be if you trigger an manual review. I'd imagine when you do it often enough, someone will be salty enough to submit a support ticket to get someone to take a closer look at your account.....

Under normal circumstances no. However you will be placed in the Low Priority Queue for repeated and consistent leaving until you can stop leaving games. More information: https://support.riotgames.com/hc/en-us/articles/201752714-LeaverBuster-FAQ