How long does it take for oyster shells to help?

We have an old hen that I have noticed lays ... well very thin shelled eggs. I gave her some oyster shells by hand today, a decent amount, but how long would it take to have effect? Normally they just help themselves, so it's not an issue...

Any idea?

This is a good question, i have wounder the same thing, all I have read nothing has said, all I know is that that say that ACV (apple cider vinegar),the kind with the mother in it, will help them adsorb the calcium, it is the same for humans.
Reg.

and I know that sunlight is necessary for them to utilize the calcium in the oyster shell somehow..
could be being older she has depleted her store of calcium and will need extra?