About the time the eye tattoos last, it depends on the material you use. However, the general time is about two or three years. It will of course fade and the color sediments will be absorbed by the eye cornea. Not everyone dare to try this. And it is not safe and effective.
Usually, if you take some eye tattoos , they will last forever. Our skin is the canvas of body art. Tattoo is just like drawing on our skin, it is permanent and can't be reversed easily.Human skin consists of three layers: epidermis, dermis and hypodermis.Epidermis is the surface of our skin and it is replaced by a new layer of cells every month. If tattoo pigments stay in this layer, they would disappear every month. Actually, as the needles of tattooing instrument hurt your skin and make pigments go into your tissue, those pigments stay in dermis, which doesn't "update" monthly. You may feel hurt when you take some eye tattoos. Itdepends on the part where you tattoo onto. The more delicate the area you choose the more pain you will have.
I think the eye tattoos is the same as you the tattoos on the other part of your body. It will last forever if you don't try to remove it. But even if remove the tattoos, it will left maybe not very visible scar. So, i strong recommend you not have your eyes tattoos if you don't want the tattoos accompany the rest of your life.
Yes, it may last for the whole life time. You know the eyes are very subtle and exquisit, once you take tattoos on it, it is hard to get rid of it or hurt your eyes. It may be a little danger and risk to get such kind of tattoos. However, it is very cool and attactive to people. I heared that, at the beginning of the tattoos in eyes, you will feel just like the ants bite on your skin, it sounds strange! How do you feel?