I read somewhere that at least 6ft from the ground or 2ft above the ear level. Positioning is also important... for DFS, there are those who like it on the side than at the rear end of the room. Try to experiment first without mounting it permanently and hear/see if what position do you like. Goodluck!
Yeah, principles on speaker placement talked about here come mostly from experiences of enthusiasts.
For me the sound experience is best when the fronts, center and surrounds are at the same level with my ears, that is:
fronts/center ---------- ears ---------- surrounds
For me, this set up allows smoother flow of the fly around effects