Voter Purge Rules

Can someone tell me how Connecticut deactivates voters? I know that if they get a "Return to Sender" when mailing voting information, that will cause a deactivation. What about expiration? I talked to someone at a Voting Registrar office and they said people are getting deactivated because they didn't vote 4 years ago. What is the rule about that and how far back does it go? 1 presidential election? 2 presidential elections?