TynanW said it, but let me add on: I was certain that I was losing my hair because I looked at my pillow in the morning, and saw anywhere from 30-60 hairs. Then, 3.5 months after being on propecia, I saw only 10. Then, a few weeks later, I saw only a few.
It isn't a good thing to keep counting your hairs, but it doesn't hurt to monitor once every week or two for the progress you are looking for, which is proof of maintenance.
Good luck.