I think a breed, type, or even a strain, is pure when it breeds true to type for whatever the breeder sets as his ideal. I also don't think you can say it takes x number of generations to get a pure line. While one breeder may advance a line or breed very fast using quality animals and making informed breeding decisions- others may breed for years-a lifetime even, and make little to no improvement to their line/breed. So, there are too many variables to say purity can be had in a certain # of generations. Some people are constantly advancing, while others may be doing the same dumb shit for ye