Last year, my first with chicks, I gave provided free choice grit at about a week old. I plan to do the same this year.
I used chick grit last year until they were 12 weeks old. Then I switched to #2 size grit. They sell it in four sizes. #1 is for chicks. #2 is for all of chickens over 12 weeks old. #3 is for large/standard breeds when they reach full size. #4 is for turkeys. I selected #2 so that I could buy 1 50 lb bag. It was only $9.00. The people at the feed store said this size was fine for their entire life (after 12 weeks). So far I have used maybe 1/4 of the bag for my 15 pullets. They are almost 1 year old. I suspect this bag will last me at least another year even though I am adding 25 more chicks this year.