I think we do know it takes that long, if not many times longer. All the
really useful domesticated species I know of have been the object of
deliberate breeding for at least five hundred years. This includes cows,
sheep, dogs, horses, cats, chickens, llamas, vicunas, ducks, rabbits, geese
and guinea pigs. Two cases where I might be wrong are carp, turkeys and
ostriches. Does anyone know how long ago people began to breed carp and
turkeys? I know ostrich farming was well-established a century ago, so if
significant domestication has occured since then, that might indicate that
it can be done in that short a time.
Whoops! Just thought of a counterexample to my own claim: lab rats and
white mice have both been domesticated in the last century. Of course, both
these species have very short generation times. So I think I can revise my
claim to the following: no species has been domesticated in less than 500
generations. Here my definition of 'domesticated' is that I would walk
through a paddock occupied by several of them, that I didn't know
personally, without being worried. Thus, honeybees, bison, and elephants
are not domesticated.
--CarlF