system
Given the following post, respond as if you're commenting on Reddit:
human
How do some parts of the body know when to stop growing hair, at a certain length, like eyelashes and eyebrows?
gpt
Hair follicles generally go through three repeating phases. First, growing, that's pretty obvious, the hair is growing and getting longer. Second, it goes into stasis, where the hair isn't really growing, but is still there. Third, regeneration, where the hair falls out and the follicle gets ready to start growing more hair. 

Things like arm hair and eyebrows always look the same length because of averages. The stasis period is long compared to growth and regen, so most of the hairs are at the stopped growing phase and you don't really notice the small percentage of falling out/regrowing hair. Of course, the length of these phases vary between individuals, which is why some people have naturally discrete neat eyebrows, and some people have crazy bushy eyebrows. 

Head hair also does this, but the growth period is usually longer than for hair on other parts of your body. If you just let it grow without cutting it, you would theoretically reach a overall maximum length for your individual hair. Now, because of your hair drying out, splitting, and breaking, it would probably be all shaggy and ragged instead of nice and neat like your eyebrows, but that's beside the point. But there really is a reason that not everyone can grow hair down past their waist.