A hen will normally stay fertile about two weeks after a mating. Sometimes less, sometimes more. Many breeders use three weeks before they assume the eggs are fertilized by the right rooster. There have been reports on this forum of eggs being fertile about 3-1/2 weeks after a mating, rare but evidently it happens. So three weeks and you are pretty sure, four weeks and you are real safe.
It normally takes an egg about 25 hours to make its way through the hen's internal egg laying factory. It can only be fertilized in the first 15 minutes or so of that journey. Assume a mating occurs on a Monday. Monday's egg will not be fertile. Tuesday's egg may or may not be fertile, depending on what time of day Monday the egg started its journey and what time the mating occurred. If the mating occurred on a Monday, Wednesday's egg will almost certainly be fertile.
A rooster does not necessarily mate with every hen in the flock every day. Notice I said after a mating, not after they are put together.
Hope this helps.