All chickens need some calcium just for body function. That's why Starter, Grower, Flock Raiser, and all the other feeds have some calcium in them, normally around 1%. Hens that are laying need a lot more for the egg shells, so Layer normally has around 4% calcium.
Some of the calcium they eat is not processed by their bodies. It passes straight through and out the rear end. That makes compost made from chicken poop even more valuable in your garden, many plants need calcium to produce healthy produce.
Chickens can get calcium from different sources, especially if they forage for some of what they eat. That calcium might come from certain plants, some of the creepy crawlies they eat, or if you are in limestone country from some of the rocks they eat and use as grit. They can get it from the chicken feed, oyster shells, or crushed egg shells. How much calcium they get and if it is enough will depend on how this all goes together.
Between the amount that passes out the rear end and the amount used for other body functions if their only calcium source is their own egg shells it is a losing proposition. But if they are getting enough calcium from other sources egg shells can contribute.
Most laying hens seem to know they need extra calcium and eat enough oyster shell if it is offered, but some may not have those good instincts as strong as they should be. Most of the time offering oyster shell on the side is enough for good egg shells. Most of the time. If they are getting calcium from other sources oyster shell may last a really long time. If they are not getting much calcium from other sources they can go through it fairly quickly. I have 11 pullets just starting to lay, my oyster shell levels have been dropping pretty fast.
Feeding egg shells back to your hens will help if you can get them to eat them. Mine seem to not like them very well, but they always have oyster shell. For the short term it will help some but I'd get another bag of oyster shell at the next opportunity.
