How long a plant can survive without light depends on the species. I recently did a week long blackout to protect some fry that were born in the tank (adults hunt fry by sight, so if you do a blackout until the fry are large enough to not get eaten, you can save them). During the weeklong blackout my Ludwigia peruensis and Proserpinaca palustris both turned to mush while the Myriophyllum pinnatum, Lysimacchia nummularia, Ludwigia repens, Ludwigia ovalis, and Hemiatus callitrichoides survived.
I personally use a Lithonia four foot shop light on my four foot long tank. You can buy one for $20 and get full spectrum bulbs for it for $8 from Home Depot. I rest it right on top of the tank.