
Well based on Bruce Catanagh's experiment with using corgi to get Bob tailed boxer, and having them accepted on the breed register once he had bred back to boxer for four generations I would think a dog would have to have 3 full generations of the new breed behind it.
A recent post on her outlined the situation with the Irish KC and Jack Ruyssels, and it would seem that once there are 3 generations of known ancestors that the dogs are fully accpted, and earlier generations are on a development register.
They would of course have to breed to accepted type (and most standards have accepted variations).
With my first breed the BSD, there was a dog that was assessed by a champ show judge as being a pure Groenendael, and was used in the early lines hear with parents unknown, and registered as Noir De Rhodesia, and was in the fifth generation behind the bitch I owned born in 1988.