How do I Calculate Full-Time Hours For Employee Who Takes Time Off?

We have an employee who works full-time. This employee takes off from time to time. There will be 2 months she works full time and then she might decide that she’s taking a month off to go to Mexico to visit family. Then she come back and wants full-time hours back. A month later she takes off to Mexico again for 2 weeks and then comes back. She is full time when she is here but leaves frequently. What is the requirement for employees such as this one?