First of all: “FTE” is however you define it at your practice. You don’t have to use an external definition; use whatever your practice typically uses.
For example: Our practice says that a full time employee is one who is regularly scheduled to work 36 or more hours in a calendar week. This definition is primarily used internally for benefit computation: for example, we pay 90% of health insurance premiums for full time employees, but 50% for part time employees.
So now you have to drill down on the definition of “regularly scheduled.” If Wanda was taking a week of vacation the week of 5/31, she technically worked 0 hours that week - but that doesn’t make Wanda part time or no-time. She usually works 38 hours a week, which makes her FT, but she was enjoying well-earned time with her family. Or let’s say the week of 5/31 had a holiday or something else. Or maybe it was a really bad week and Bertha, who usually does 30 hours, worked 42 hours that week. If that’s not a typical week, I wouldn’t say Bertha is “Regularly scheduled” as a FTE - we had a shortage and she offered to work extra to help us out.
When you think about lookback periods (i.e. how long does someone have to work a particular schedule before it becomes their “Regular” schedule), I don’t think there’s a single right answer. It’s not even a question you may have had to answer before. We have sort of used 4 pay periods (about 2 months) since 2 months is a typical maternity leave (for our staff moms) and we pay for health insurance for 4 pay periods for our usually-full-time-if-they-had-not-just-birthed-a-human staff. (Folks who want to take 3-6 months off can do that but we don’t consider them FTEs at that point.) But again, this is just how one practice in Podunk TN does it – YMMV.
If your practice doesn’t have a precise definition, you can come up with something that works for you, and just be consistent.