from what i have noticed in all my experience so far, if an egg goes cloudy, white or yellowish and not the clear, clean looking colour then they have fungused/infertile/dead.
i think it is usually pretty quick for infertile eggs to turn white.
As soon as an egg is fertilised it is a chemical reaction like boom of cellular activity. Everything happens darn quicky in there. The egg that is not lucky enough to be fertilised is stagnant. it will do nothing so the comparison should be noticeable fairly early.
I would think that after 15-16 hours the eggs that look clear and healthy are definitely good and fertile.