If someone is inactive longer than:
-few days - that means he don't like this game - if u like it, u will play it at least few times a day,
-2 weeks - he doesn't play the game because it requires you to log in everyday,
-30 days - already quit the game.
So auto-kick inactives longer than 30 days should be standard. If they start playing the game again than they can rejoin any alliance.
Also what is the point of keeping in your alliance someone who play every few days. He can't donate troops, chat, take part in wars - simple put he's useless to the rest of the members
So if not auto-kick inactive members after 30 days, then at least auto-promote any co-leader to leader if leader is inactive longer than 30 days.