How soon are eggs fertilized?

[she-earl]

I am thinking about separating my chickens into pure groups. I know that after 3-4 weeks the eggs will be considered pure. My questions is - will the eggs be fertilized "immediately" or will I need to wait a couple of days before collecting the eggs for hatching?

 

[al6517]

The rule of thumb is 2 day's as the first day there is a unfertilized or other fertile egg in the vent and the next one in the shoot to develop will then be fertile, by the new Rooster.