That's a good question but unfortunately there isn't a good answer any better than "it depends". Hens need calcium for their egg shells. They can get calcium from chicken feed, plants they eat, and creepy crawlies they catch and eat. If your native rock is limestone they can get plenty of calcium from the rocks they eat for grit.
If they need it the seem to know it and eat offered oyster shell, If they don't need it they don't eat a lot of oyster shell. They may eat up a bag in a few months (depending on how many you have). Or a small bag may last years and never go bad.