For adults they grow (on average) about 0.02 inches a week (or almost half a millimetre). That works out to 2.3 cm (or 0.9 inches) per year. That's about the same pace that continents move, by the way.

If you're thinking that your nails definitely grow more than that -- you're probably right. Young people have faster-growing fingernails than oldies. Fingernails also grow more during the day, in summer, and among people who live in warmest and sunniest climates. Men have faster growing fingernails than women. Children have the fastest growing fingernails of all. In fact, a child under about 12 can lose the very tip of their finger, and the finger tip will grow back (nail and all).

The fingernails on the longest fingers (the middle fingers) grow fastest of all. Also, the more the hand is used the faster the nail grow (because of a better blood supply). Good health promotes faster fingernail growth.

Toenails grow between a quarter and half as much as fingernails.

 And the more milk you drink, the faster they will grow. At least they do for me.