I've had to replace nuts. Just did one on my Martin D-18 E Retro, but it had been buggered up by the previous owner. Most bone nuts will last a long time. The old plastic nuts would wear fast, and the brass nuts popular in the '70s didn't hold up. I've heard of graphite nuts wearing, even those locking nuts on Floyd Rose guitars, but this is kind of a mystery.
Some times they can fix it with some powdered bone or baking soda and super glue to build up a slot.
Try using some of the Big Bend's Nut Sauce to lube the grooves when you change strings. This will DRAMATICALLY reduce friction and reduce wear of the nut slots. It's good on saddles, too. I've used it for years and it gets my strongest recommendation. A must for vibrato-equiped guitars.
Good luck.
Bill