I was hoping for at least a two week fertility period, but had two occasions that didn't fit that scenario.
I recently set a test hatch of eggs from an active unrelated breeding pair who have been together since hatch and are right at a year old.
I dated the eggs as I collected them. Granted it was a small sample, but every other day was infertile; only half of the eggs developed.
Of those that developed, they all hatched and were nice and robust.
A completely different coop and breed - I lost a rooster in a storm. Naturally the hens were off for a few days, but when they laid again I collected for a week and popped them in the incubator, not one was fertile. So it goes...
btw - was that your Father's birds and the bees explanation, or was it strictly about poultry? If so, what were his interests in poultry?