how often should a single hair be falling out?

[ikibah]

I have a couple of hairs that are noticeably lower than the rest of my hairline so it's easy for me to gauge when they're missing/fall out.

I was under the impression that the average cycle for a hair before falling out can be a couple of years. - however I've noticed that some of these hairs will fall out and regrow a couple of times over the span of a year. Is this normal?