i grow with a 250 hps and the heat is not too bad.. i would say a foot to eighteen is plenty... i think the old rule of thumb about holding your hand below the light and at which point it it comfortable for you to hold it there for a few minutes without getting too hot for you is a good one.. if it is too hot for your hand, then it will also be too hot for your plants imho..
you should check out the 250 hps club.. lots of good info on it for peps who use these lights as well as people who grow with them and who are more than willing to give sum good advice..