I have a seperate chick trough that has oysters shell in it. They pick at it when they need to. It usually lasts a long time. If I take it out for a couple of weeks they go nuts about it when I put it back in.
I have been raising laying hens for years, I have never, ever, bought oyster shell. We save the egg shells, let them dry out in the chicken bucket, grind them up and put them in the chicken feeder. Free. The only caution I would give is make sure the shells are ground up very good so they don't look like an egg. The last thing you want is for your flock to start eating their own eggs.