Personally, I don't think there is a hard and fast rule to follow. Pretty sure I've never kept a bird 3yrs up to this point. If I did it was a hen. However, I've finally got 2 W and 2 BW males that I think are pretty decent so I'm gonna use them for at least 2-3yrs - unless I happen to get another male that is really outstanding. But there is NO way I'm keeping 75 cockerels and raising them up this year like I did last.
I know Kenny Troiano has written a couple of times in the PP about holding onto and breeding the dickens outta a bird that is "prepotent" in particular areas. I haven't been able to figure out with any specificity exactly any particular traits that are prepotent in any of my birds but I've got about four specific areas I'm wanting to improve on now and if I can identify that in a particular bird, then I definitely would keep it.
I think my main problem now is that I am breeding too many birds. For example, one of my pens now has 10 full blood sisters being covered by one male. Not all of those sisters are going to possess and pass on the same genes. So I'm hoping that as I move forward I can select even more carefully even amongst those and by doing so reduce the number of birds I have to breed in the future.
For example, I have definitely noticed that the more porous eggs do not do as well as those that are not. But I haven't been able to determine yet whether that's totally because of the porous egg shells or just not being fertile. I do know that I see more bloodrings in the porous shells but I also tossed a lot last night that weren't fertile. Why weren't they fertile? Could very well be that it's because I have too many gals in with the one rooster.
Unfortunately, because of what happened while I was away a few years ago, and basically having to start all over with figuring out who's who in my flock, it may well take me a few years to figure it all out again. I am thinking though that the porous eggshell problem will rectify itself. I mean, if they aren't hatching and only the good shells do, it's gonna fix itself.
Then there is also the size of the egg, the color of the egg, and the productivity of the hen to consider. And that's just dealing with the egg, not to mention Type and Color.
I would suggest that by running at least 2 different lines, one should not have to go backwards and start over. I've not gotten down to 2 lines yet. The least I've been able to attain was four. And perhaps because I've always had multiple lines, I've always been able to move forward without going backwards. Meaning, I've never had to go back to a cockbird or a hen because I've lost something. Each year I've managed to make some improvement in the overall presence of my flock.
Even a few years ago when I had to start over. Due to my absence and none of the eggs I had in the incubator being toe-punched, I started over by taking my 2 best W males (didn't have any BWs that I wanted to use) and mated them to all my females. Since I was basically starting over and using some pullets without an ID, I just numbered the first W male's eggs as 1-3, and then the second W male's as 3-6 based solely on the egg color. #1 being the best blue and #3 being white.
That gave me a place to start. Then the following year I started selecting on Type & Color again. This year I'm breeding 2 W and 2 BW males with 1 of the W & 1 of the BW being from the same line. And I've got 4 lines of females with some overlap as well. Next year I am hoping to get back down to 4 lines and the year after than hopefully I'll be down to 2.
One other thing I would say is that I, personally, would never get rid of a bird just because it's not a double muff. Heck, if I had an outstanding bird that was clean-faced, I'd breed it. Take that clean-faced bird and breed it to a single or double-muff/bearded bird and you're gonna get muffed/bearded birds. To me that's such an easy fix that I don't see any reason to get rid of it if it's exceptional in other areas.