This site has a lot of links to loads of questions. I was hounded on FMLA when I was on it for about two years. The company will keep track of hours and days taken, cause they don't want to give you one more minute than is necessary.
When I was on it, they could, and did re-certify me many times. It was legit so it just cost them more money each time. At the time, the employer can re-certify you or your family member as many times as they'd like.
U.S. Department of Labor: Compliance Assistance: Family and Medical Leave Act (FMLA)