it depends on the size of the lamp, and the wattage of the bulb. I would set it about 18'' above the tub, and go from there checking periodically and adjusting.
heres my stand alone tub that i use for QT:
i cut a hole in with a soldering iron, and hot glued some screen over the hole
and the light i use is for a 10g aquarium, with the double socket in the midde, and i have one 15w bulb in the front. as long as my room doesnt fall below 65, my temps are fine
if you use a normal dome lamp, it should be fine as long as you have enough time before you leave to figure out how far away to set it up.