If your obligation to repay is based on a contract, as you indicate, then the employer can take legal action against you any time within the "statute of limitations" (time to sue) for breach of contract. In your state, that is four years. Since the time period during which they can bill you is, for all practical purposes, defined by the statute of limitations--since after they can no longer take legal action, they could not force you to pay a bill--they can effectively bill you up to almost four years after you left employment (since they'd need to leave themselves enough time to sue if you do not pay).