what kind of chicken, a silkie or a cochin will need a lower box then a leghorn or langshan. average is 18 to 24 inches, but it needs to be lower than your roosts, it helps discourage your chickens from sleeping in and on the nest boxes, but you will always get a hen who wants the nest box not the roosts
We have ours anywhere from 12" from the floor up to around 3' from the ground. The main thing is to make them lower than your roosts or the chickens will be trying to roost on and sleep in the nest boxes.
You can see pictures of some of ours in the links in my sig below.