Grit for grinding stuff in their gizzard and oyster shell for a calcium supplement are two totally different things. I'm not sure where you are, there is a bit of a language barrier here. In the UK and possibly other places they call oyster shell soluble grit and the stuff used to grind up stuff insoluble grit. In the States, grit is the stuff they use to grind up stuff while oyster shell is never called any kind of grit. This sometimes causes confusion on this forum.
Since chickens don't have teeth but eat things that need to be ground up they eat small rocks that act as teeth in the gizzard. They can use various rocks for that. The grit you buy is usually made from granite, it's a by-product from a granite quarry. Granite is very hard and the larger pieces can last a month. They can use whatever rock is in your soil but depending on what your native rock is it might be softer and get ground up pretty fast. Normally if your chickens have access to the ground they find their own grit, but there are some soils that just don't have rocks.
Laying hens need a fair amount of calcium for their egg shells. If all they eat is Layer with about a 4% calcium content they will get enough calcium from that. But it's not what is in one bite, it's the total amount of calcium they average eating in a day. If they are eating other things that are low calcium they may need a calcium supplement. A standard inexpensive easy way to supplement the calcium is to offer oyster shell on the side. That way the hens that need the extra calcium can eat it and the chickens that are not laying (young chicks, roosters, or hens not laying) don't eat enough to harm themselves.
There are other possible sources of calcium. They can get some calcium from certain plants, some creepy crawlies if they can catch them, or even some rocks if your native rock is limestone. The way to tell if they are getting enough calcium is to look at your egg shells. If they are hard and thick your hens are getting enough calcium from somewhere. If they are thin or soft they probably need a calcium supplement.
Oyster shell will not work to grind food in their gizzard. Part of that is that it crushes to a powder too easily but a big part is that it dissolves in the digestive juices so it can be absorbed by their bodies. Their digestive juices are acidic and that dissolves the oyster shell pretty darn quickly.
I don't know if you need to add insoluble grit to grind their food or not. If they spend some time on the ground and your soil has rocks in it, probably not. You'll sometimes see them pecking at the ground. One of the things they are looking for is rocks to use as grit in their gizzard. Mine seem to do that a lot when they are taking dust baths.
I consider it good practice to offer oyster shell on the side, whether they need it or not. It generally doesn't cost much and never goes bad. If they are getting enough calcium from other sources that oyster shell may last a long time. If they need it then it will disappear faster.
