There's an airline at my home base who holds about 20% of the market, but the user hasn't been active since May 6th. His marketshare is quickly decreasing, but he's still using up valuable slot capacity (FRA is the airport with 2nd most used slots percentage) and capacity. Is there any procedure or rule on this?
No. It is perfectly acceptable to leave on holidays. Slot scarcity is part of the game, and learning to play with it is part of the pleasure.