If you have a sitting hen and have decided that yes, you’d like to give her some eggs, then give her some when:
1 – you have convinced yourself that she means business. Since broodies are very dedicated and barely eat or drink while sitting – you don’t really want to wait more than a couple of days.
2 – she is in a place you like or at least are willing to work with
3 – you have the eggs you want her to hatch.
If the last point does not matter, just give her up to 12 eggs all at the same time, or, anticipating you might have a broody, you have kept some pre-collected eggs from your favorite layers and use those. Just remember it is best not to use eggs that are too old. (none older than 2 weeks) & store them cool at room temperature while waiting. I just gave my laid one all the eggs laid that day.
It is best to give her all the eggs at the same time. Having done it differently, it complicated hatching a bit. Also, it helps to mark the eggs if there is any chance another hen will have access to the nest as there WILL be more eggs then and you need to know which eggs to remove.
It IS a good idea to have a back-up plan – as in: an incubator ready in case she is not going to see it through or kills the babies as soon as they hatch. This is if you do not know her. Once a good broody and mama, you can trust her with even special eggs. One the other hand, they say if a hen does not know what to do, or is a bad mama – don’t trust her with eggs another time. My guide says you may give her 1 other chance if she was just a very young hen …but I’d watch her carefully.