Depends also on size & type of grit.
They eat much more of the smaller size. They like it better b/c it's easier for them to eat, but it grinds down faster.
Also noticed a difference when I gave them grit from different places. The one from my feed store is granite. I think the one from my gardening store was limestone. I bought it because I was in a hurry. But they seemed to go through it quicker, I think because it was a softer stone.
For context, i live in Oregon where I have a lot of clay soil & larger river rocks. Not so much "grit". I have 3 hens who forage anywhere from 1hr to all day everyday, depending on my work schedule.
If your girls are eating that much, I'd keep it out for them. Use a taller feeder like the pail @GC-Raptor shows to refuce waste. But even if they fling it around, they can scratch around the ground for it