All of your hair may need cutting because you prefer it at a certain length, but if you stopped cutting it, each hair would get to a certain length before falling out. I have seen a woman who had never had her hair cut in her life yet her hair only came to just past her shoulders. That was the natural length of her hair and that's as long as it would get - it doesn't keep growing indefinately. The same goes for your eyelashes. They won't grow to the point they are in your eyes or causing an obstruction to your visiion - each lash will get to a certain length and then naturally drop out and a new lash will start growing in its place.