When a Clan Leader hasn't logged into Clash Royale for 21 days, the process of selecting a new Leader begins. Inactive Clan Leaders are sent messages and notifications encouraging them to login and manage their Clan again. The Clan Leader has 14 days to come back to the game before the status is given away. After a total of 35 days of inactivity, the Leader role is given to a Co-leader or an Elder of the Clan.
Clan members and the Leader will see messages about the process in their Inboxes:
21 days of inactivity:
32 days of inactivity:
35 days of inactivity:
The new Clan Leader will be the longest-serving Co-leader. If there is no Co-leader, then it will be the longest-serving Elder who is promoted to Clan Leader. A Clan Leader can willingly give up his Leader status by logging in and promoting another Clan member to Leader.