Really quickly. It takes aboout 25 hours for an egg to go through a hen's internal laying factory. It can only be fertilized for the first 15 minutes of that journey. After those 15 minutes, it is too late.
If they mate on a Monday, Monday's egg will not be fertile. It's 15 minutes were on Sunday before they met. Tuesday's egg might be fertile, depending on when the mating occurred and when it started it's journey. But often, that egg would not be fertile. A lot of hens lay eggs early in the morning which means the rooster will probably be too late. Wednesday's egg would almost certainly be fertile.
Note that this is after mating, not just an introduction. If it is only one hen and one rooster, the mating is usually pretty quickly after they meet, but if he has a flock, it could be a few days before he spreads his talents to all of them.
After a mating a hen can lay fertile eggs for a while. It can be for as few as 9 days after a mating, but most people count on two weeks. She can lay fertile eggs for three weeks or more, but two weeks is more dependable. If it has been a month since she was with anothe rooster, the Silkie wil be the father of any chicks that hatch.