The best time to give a broody eggs is 1 week in, when it's obvious that they're committed. I wouldn't give them eggs if this is their first time going broody - let them get used to that hormone rush first.
If they are a broody breed, be aware that they will be 'really' bad at taking care of themselves. You may need to force them off the nest twice a day to get them to eat, drink, and dust bathe whether you've given them eggs or not.
It will take them a few months to cycle back around after a long brooding session. They may go into a post-broody moult as well, so don't expect these girls that are currently brooding to be the ones to hatch your eggs.
Make plans to brood and rear artificially now, especially if you paid a pretty penny for shipped eggs.
I have an older broody that's 2-3 weeks in whose a great mother, but I won't be giving her any if the eggs I'm having delivered next week - it would be too much to expect her to sit another 3 weeks on them. The longest she's brooded air is 5 weeks and they can lose a lot of condition in that time if they aren't taking care of themselves. - this goes double if you give them eggs. They 'know' and a young broody can downright get obsessed and depressed on their first clutch, refusing to leave the nest for self-care.
First time with eggs, my girl started throwing all of them on day 19 (one had quit but I don't think she knew which one) I had to sit with her for a whole day to get her emotions in order so she could finish the hatch.