I don't know if an immediate two week ban is the answer, all I know is that AFKing definitely needs to have more severe punishments.
Everyone and their mother AFKs like there's no tomorrow. You have people ragequitting, people going under tower to text for 2-3 minutes at a time, people who AFK just because they know the punishment isn't that severe if they were to even get punished at all.
Personally I'd aim for something like the chat restriction ladder. First offense is (relatively) minor and they gradually escalate. This is my only large complaint about the system, AFK rules need to be more strict.
Edit: And the only reason I don't think a 2 week should be the first step is because accidents do happen.
Of course I don't want anyone to AFK but sometimes it's intentional, sometimes it's not, and Riot would have to balance the system around that fact.