Put out regular grit at any point and take away chick grit once they aren't taking any. You can just mix them together. If you free range at all it doesn't make much difference unless your are somewhere that is mainly sand or clay, etc and not a lot of pebbles and such.
There's an article on site that states when they get larger - at least 8 weeks old to make the change. The only difference in purchased grit is chick grit is a lot finer.
IMO - use chick grit until they are getting nearly grown size - when their beaks and mouths are large enough for the adult grit - buy a bag of both to see the size difference.