I am another who thinks egg shells don't provide quite as much calcium as oyster shell. I offer it in a separate container and they eat layer feed, just nibbling on the oyster shell when they need to. But I do like to boil up any older eggs I have in the fridge, crush 'em up shells and all, and give those to the girls a couple of times a week. Granted, with a lot more chickens I probably have more older eggs - eggs that I don't want to sell because they are more than 2 weeks old.
For me the bonus of doing this is that they get the egg shells, but they also get the protein from the eggs and I like that this time of year since many of mine are currently molting. I have never had a single "egg eating" issue doing this. What I toss in their pen has no resemblance to what they are leaving behind in the nest, so it hasn't caused any bad habits. I usually boil them up and when they're cool I put them in a plastic grocery bag, twist it shut and pop it back in the fridge. When I'm ready to feed them, I tuck that bag into a second grocery bag so I have a double layer, lay them on the countertop, and start "smushing" with my hands. Once the shells have been cracked, it's like kneading bread. Doesn't take long and they are all crushed up and ready for the girls. They love 'em.
I feed mine old and any less than desirable eggs as well. Hard boiled, cooled, and then into an old blender. Sometimes I'll mix in some veggies or other treats if they around, along with any egg shells. Reduces everything to crumbles so there is no resemblance to a whole egg. Once a week normally. But in my case the calcium in the shell is a bonus, the main purpose is to recycle the egg protein to offset feed.
To answer the original question hens should have access to a calcium source at all times, unless their diet specifically has enough. I feed layer feed most of the time that has calcium, they also have oyster shell available. If you are providing the egg shell as a supplement leave it in a bowl they have access to, they will self regulate.