I'd say 10"-20". They would work fine much higher up but then everytime you look at the tank you get blasted by the light which is really distracting. Depends on the layout of the tank and the plants your trying to grow too.
I'm going for higher light plants.. Id like lots of reds. I'm going to try some L. Glandulosa, Ammannia Gracilis as well as some bushy greens and a nice thick bright green foreground.. not sure which yet, still in the planning stages. There will probably be a mossy rock in there somewhere too..