What do you mean "amount"? Hours or light intensity (not sure of the unit lol)?

If its light intensity, depending on the space, you could get away with ~100W of CFL. Could get lots of like 20W bulbs, or just buy something like this: (sorry for the other unnecessary things in picture) http://sog.open24x7.biz/catalog/imag.../img2-1091.jpg

Basically a single 125W CFL.
Costs about 100NZD(70USD) here, not sure where/if you could get one in the US tho.

As said above, prob best to keep it at 18/6.