How long does one human sustain a Wraith for?

In Common Ground, Todd says that the life he took from Sheppard (which looked to be at LEAST 10 years) would only sustain him for a few hours. Does this mean they have to feed off a human every day? Or maybe they get less nourishment until the final bit of life force is taken, and that is much more. IT seems like it would be impossible to sustain any decent sized population of Wraith, even with their pretty sustainable harvesting methods. Every Wraith requiring hundreds of humans every year? Hell, even if they make it to Earth, they won't last long.