The Fat-Tailed Sheep on the First Fleet; Australia's First Sheep

If we're talking sheep, then I have to put out a shout for my local breed - the Romney Marsh sheep (the sheep that conquered the world!) - https://en.wikipedia.org/wiki/Romney_sheep

I was always curious how long it takes that type of mutation to become a distinct strain (e.g. how many generations from the first sheep herder that saw a sheep with a fat tail and decided, either consciously or otherwise, to breed it and continue to select offspring with fat tails until it no longer required selective propagation to persist).