For standard chickens, about 16 in off the floor. A rail of some sort in front of the boxes will help the chickens when deciding where to lay their eggs. My hubby made a fold down rail for easy cleaning.
i put bottom of my boxes the height of my roosters head
that way the old roo can't stick his head in there to mess with my layers while they are "working"
& as Chickflick said they need some kind of rail or board so they have a "landing srtip"