Your fundamental question was how long does it take to stablize a line. There is no short answer. I made a cross of Silver Laced Wyandotte X blue egg laying Brown Leghorns with the intent to stabilize the blue egg trait in a Silver Laced Wyandotte background. I'm working with 5 major genes plus the oocyanin gene. There are at least 20 or 30 more minor genes in play. I started in early 2014 and have 10 years of work done so far. I am finally seeing chicks hatch that look like what I want. I also have very pretty blue eggs from the current generation of hens. With a bit of luck, I'll have the traits stable in another 5 years. Could it have been done in less time? Yes, but I would have had to use intense inbreeding which would have been a problem over the long term. Taking more time and maintaining more birds permit selection without losing vigor and performance traits.