It depends a bit on tank size, but with good reflectors, and good bulbs (T5, PC or MH) I'd say
<1.5 W/gal low light
1.5 - 2W/gal medium light
>2W/gal high light
With less optimal reflectors and not so good bulbs (T8 or T12) maybe
Wattage is just one thing, what matters is how much light makes it into the tank.
Of course, what is medium to me could be a totally different thing to you, so that's all extremely subjective.