It won't be anything fast, it would take like a month to totally decay. Player would have to maintain it regularly if he/she wants to have their dominion. Higher-level dominions would degrade into lower counterparts until there's nothing left of it.
Stuff on the "plot" won't be removed, only dominion stone so after it decays everything's up for grabs.
The reason behind this is to stop people from making big dominion, then forgetting about the game - the plots made by MIA players won't clog up the server and could be then used by more active players.
Again, the decay process shouldn't be fast, it should take like a month per dominion "level" so people who only have time to play every other week won't lose their entire legacy.