just depend but usually roosters mature sooner than the females, My heritage breeds just started laying and we tossed first 3 days of eggs in incubator as they are on conservancies threatened list, and we had 1/4 that is actually fertilized. you would probably be best but chickens didn't read the rules book to know the rules to wait a month after she starts laying or see if anyone around has some fertilized eggs if she truly goes broody, a true broody will not be picky about the eggs under her and raise them as her own. Mine started with first egg but had to get some practice in obviously lol