Well the scale just refers to how far down the breeding line the fish is from its wild ancestors. I suppose that you could keep it going for as long as you could trace it (ancestory.com lol?), but I have only ever seen F0 (wild caught) and F1 (their fry) labelled in a LFS. Unless I was going to buy the fish for pure breeding purposes, then I would buy F1 as they are used to being in a tank and their genes and colours will not be watered down as they can get further down the genetic line. F0 fish are also meant to be quite jumpy and hard to acclimatise. I suppose that is understandable as they have been dragged from their wild home into someones front room.
I have a two F0 Plecs and one F1 Severum. Oh and and all of my Hillstream loaches will be F0.