First I’ll give you some information you did not ask for, but it might come in handy to know it.
It takes an egg about 25 hours to go through a hen’s internal egg making factory. That egg can only be fertilized in the first few moments of that journey. That means if a mating takes place on a Sunday, Sunday’s egg is not fertile. Monday’s egg might be but I would not count on it. Tuesday’s egg is almost certainly fertile.
The last part of the mating ritual is that after the rooster hops off, the hen fluffs up and shakes. This fluffy shake gets the sperm in a special container near where the egg starts its journey.
That sperm can stay viable for quite a while. Most of us assume a hen is still fertile two weeks after a mating. So look at two weeks as a general minimum. Many breeders wait three weeks after a mating to give enough time for the sperm to not be viable, but there have been cases where an egg was fertile even a couple of days longer than three weeks. So three weeks is pretty safe but four weeks is safer.
A professor that teaches poultry science and is an expert on chicken reproduction said that the special container to store the sperm operates on a last in – first out basis. That means the last rooster to mate with the hen will father her chicks. I understand it would take a leap of faith for you to trust a stranger over the internet in something like that. Personally I trust that professor. When I last saved eggs, I took the roosters I did not want to be the sire out on a Monday and started saving eggs that next Friday, trying to give that rooster enough time to mate with each hen in the flock. If you don’t trust me on that I’m certainly not offended.