Employee may be using FMLA leave for years to come–how often can we request an updated certification?

Q: “If an employee has requested lifelong, intermittent FMLA leave (worked one year, and worked over 1,250 hours in preceding 12 months), can an employer make a request for updated medical certification once per year? The employee continues to work at least 1,250 hours in a rolling calendar period.” – Anne, Minnesota