I've had my kindle for a year and the letters haven't rubbed off. However I have a cover that protects the front of the kindle when it's not in use, so there wouldn't be any reason for the letters to rub off, they don't touch anything

If you've had it for a year, you don't have the K3. I don't know if you're aware but some of the K3's had a known problem with the letters rubbing off within a few days/weeks of receiving it, just from general usage of the buttons - a cover would not have helped.
To the OP, if I remember correctly, it was only certain graphite ones with the problem, not the white ones.