
Certainly when the KC allow out-crosses to other breeds the dogs recognised as purebred have to be from the fourth generation down from the out-cross, so certainly 4 generations of breeding pure, without continuing introduction of outside blood is likely to be the least.
With modern genetic knowledge and how important it is to have a very wide base of founder animals I cannot see how a viable new breed can ever be ethically achieved.
Most new breeds accepted have been existing types/breeds, bred to a standard and recognition applied for (Border Collies, Parson Russells for example in my time in dogs), or foreign breeds.
It would require large numbers bred repeatedly to obtain the small number of animals suitable to carry on. With culling being totally abhorrent and unacceptable in most circles, breeding large numbers of pups that then need to be homed is going to be a huge ethical issue