Depending on what birds are used and what constitutes the new breed someone is making plus how they select and breed that determines how long it takes. There's no set time. It may take 5 years, 10 years, 20 years or never be accomplished.
As far as a new variety or color some can be created and breed true in two generations. Many can in three.
I've made new varieties myself. Just hatched another new one yesterday that took two years.
Its a self blue (lavender) silver duckwing pattern
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