It depends on your fementation temperature, warmer means the yeast will work faster, too warm and the yeast will stress and cause off flavors in your beer. Too cold, the yeast will stop working, and go to sleep.
It will be done when the yeast calls it quits, that's what he means.
I've had a 5 gallon batch done fermenting in 4 days.
I've had another 5 gallon batch done in 9 days.
That's all because of the fermentation temperature.