It could happen in the first few days, but to be sure, probably wait a week to 10 days. If she has been with other roosters, then you should wait 4 weeks. The sperm is stored by the female and released over several days.
This is from what I've read....
It depends largely on how many hens your Rooster is servicing.....
and how many he gets to per day.
I have 1 Rooster to 33 hens.
The eggs gathered & hatched thus far say; they have a fertility rate of 92.59% of the eggs collected on any given day.
Some may be fertile the morning after he is placed in the henhouse; but to be sure I would watch the eggs I was cooking for the bull's eye! I check all my eggs before I cook with them. When I crack them I note whether or not there was a bullseye. Not that I tell anyone else if I seen it! lol