It's a spur. Not all hens have them, though I've seen it common in my Golden and Buff Comets. I'm not exactly sure what causes it in hens, but I'm assuming it's hormonal. I haven't seen a hen spur as long as your hen's, but spurs never stop growing, so that's why her's is so long. What breed is she?
As far as what to do about it, I'd either trim or file it down a little. There will be a quick (where the blood is in a nail, beak, or spur) that you don't want to cut into, so be aware of where it is. But I would do something because how long it's getting and it's curling. (Basically just take the edge off the end will be good enough. You don't need to do anything if you don't want to. She'll be fine either way, but I would do something.)
As far as what to do about it, I'd either trim or file it down a little. There will be a quick (where the blood is in a nail, beak, or spur) that you don't want to cut into, so be aware of where it is. But I would do something because how long it's getting and it's curling. (Basically just take the edge off the end will be good enough. You don't need to do anything if you don't want to. She'll be fine either way, but I would do something.)
