Employee goes dark after FMLA leave expires–when can we terminate?

Q: “If an employee was on FMLA leave and has exhausted the 12 weeks, but has not made contact with the company (to state that they are desiring to come back to work, or to request an extension of the leave), and their phone numbers and email are no longer in service, how long is the company required to wait before termination of employment?” - Martin, Minnesota