On dedicated servers, plots are leased rather than sold outright. The lease window is configured by the server administrator.
What happens at expiry
When the renting player stops playing on the server long enough for the timer to expire, the plot "will eventually go up for rent again," restoring it to the public pool. This recycling mechanism is the canonical anti-hoarding measure on community servers.
Renewing
Renters typically renew by re-interacting with the plot's postbox before the grace period elapses. Server operators may tie reminders to Discord/web tools.
Why this exists
Land would otherwise be locked up by accounts that never log in. The rental model keeps the property market liquid for active players.