Any claim that is improved (built upon) is protected for 60 days of inactivity.
If you own a total Claim Block balance of at least 5000 then your claims will be protected for the life of the server.
If you see that a claim is 17,000+ days old and by an unowned player what happened there?
We delete players data at 180 days of inactivity. (this includes homes, inventories, enderchests etc.) at this point the server forgets the players name. As per the note above about expiry however the players claims are protected forever if they met the total claim blocks limits. (note claim blocks do not expire and you will keep them for the life of the server)