When do eggs become fertile?

Can anyone tell me how long a rooster needs to be with the hens before their eggs become fertile? There have been no other roosters involved prior to reintroduction.

Thank you

I guess it depends on whether they are getting bred right away, but assuming the roo is getting the job done I should think 3 or 4 days or 3-4 eggs should be plenty of time.

I often crack eggs until I start seeing fertile ones or if I am just hatching for myself, stick them in the incubator and find out if they start to develop...

I start collecting 2 days after and have had good results with that.

Wow, much quicker than i thought and he's well 'practiced'. Thanks Miry and Chantecler.