Progesterone is a hormone released by the corpus luteum in the ovary. It plays important roles in the menstrual cycle and in maintaining the early stages of pregnancy. It may also be involved in the growth of certain cancers.
I've been following various people's posts about the GRE for 10 months now. Some people don't study at all for the GRE, some study for a weekend, the average is about 2 to 3 months, and some study for a year or more.
This is good for ETS. Most people will probably only need to take the GRE once. But if you find yourself changing career tracks, or returning for another graduate degree later, you may need to take it again if five years have passed since the last time you took the GRE.