I'm trying to design human like race where all the children are born as girls, but later in life some of them switch to male, but no sooner then 15. Usually only the most dominant women switch, so in normal times women are majority of the population, but in exceptional situations like war or overpopulation gender ratio might be reversed.
As soon as transformation starts woman is infertile until the transformation is finished. I have chosen this in order to avoid self impregnation. Transformation is irreversible, man can't transform back into woman. The woman must start transformation before certain age, otherwise she will stay woman for good. I haven't decided on the exact age until I know how long should the transformation last.
How many years should the transformation from woman to man take?
I don't know much about biology but I'm expecting some plausible approximation rooted on examples in either humans or animals, like Frostfyre's links about genetic disorder and mice hormone experiments.