Not long. It takes about 25 hours for an egg to make it through the hen's egg laying factory once it starts. It can only be fertilized during the first 15 minutes of that journey.
Let's assume your rooster mated with a hen today, Monday. Monday's egg will obviously not be fertile. Depending on the time of day the mating took place and the time of day Tuesday's egg, starts its journey, Tuesday's egg may or may not be fertile. It is extremey likely that Wednesday's egg will be fertile.
Notice that this is after a successful mating. Although they may want to, not all roosters mate with every hen in the flock immediately after being introduced to the flock. However, with 5 hens and one rooster, I'd feel pretty comfortable that Thursday's eggs would most likely all be fertile. Heck, Wednesday's probably would be.