From their help forum: “your account will become dormant after about 6 months and removed about further 3 months on.”
If you created your account using Bigpoint, you can remove the service from your games list. This effectively deletes your account, but it remains a bit shady.
Helpful Trick: Removing online data
Helpful Trick: Remove third-party logins
If you signed up for Farmerama using a third-party login such as Facebook or Twitter, you probably want to revoke all access from Farmerama. Click here to find out how.