When is a line “yours”?

Three or so generations, I reckon, of you breeding, picking, and keeping both sides of the pedigree, before you can think of it as yours. The first couple of generations are really just building on what you were given, and if you have decent stock you’d have to work hard to fuck it up that quickly. By the third generation, your selection pressures and husbandry are going to start having a decent amount of influence on the babies you produce.