So much of this is going to depend on the strain. There are certain faults that allow one to cull early, but I have found that, with the whites, six months is the safe(er) point. By then one has a pretty strong sense of where the bird is going. Of course, surprises will continue to unfold, but they tend to be in line with the general development of the fowl by that point.
Dorkings are generally rough enough at this point that there tend to be some pretty clear winners and losers. Over the years, as they improve, it will like become more and more difficult, as uniformity and high quality are achieved.
We'll be holding about 30 pullets into the winter. I don't like to select them until late January. By that point, they're showing you their productivity and more mature frame and weight. The cull females are great stewing fowl come spring.