My birds get oyster shell as a free-choice calcium supplement in the coop. They all free range when we're home, but put themselves away at dusk, so they know where their regular food and water source is, and we go through about a cup of oyster shell every week.
We provide 16% layer feed, but consumption of that depends on how long they're allowed to range. We only range when we're home, so that usually means after work during the week and all day on weekends.
We have the odd shell-less egg every now and then; perhaps about 6 to this point since this flock started laying in December. I'm not overly concerned about it.