There is no rule of thumb as to where your nightstand should be fixed, and it depends on your bedroom set up, but generally, the best nightstand set-up is the height at the same level as your bed, or a little bit higher. If you're doing a whole bedroom refurb at once, make sure you've chosen the bed and mattress (and know what height they come up to) before you hang the floating nightstand.
Think about how you use the space - reaching for something during the night is a lot easier when you are at the same level, and it's less easy if the nightstand is below your bed.