You can use it wet or dry, sometimes it's easier to use it wet because it's going to feel like it's softer against your nails. I wouldn't say it's going to hurt or weaken your nails, it's just water (you wash dishes, your hair, body etc with it and that doesn't weaken them
) Washing your file is also how you clean it, so you have to do that from time to time even if you don't use it wet.
They should last forever, as long as you don't drop it/sit on it etc.