I added it because I have gotten a few weak shells. I hate sticking my thumb through an egg shell while I am rinsing dooky off the shell before I crack the egg or set out to boil it. I set it out after I had already gotten some weak shells, so a month or so after the pullets had started laying. I filled a 3-pound plastic Little Giant feeder and hung it. The level went down by half over the course of about two weeks, and has decreased a little bit over the course of the following week. It'll be a while before it's empty.
Whether they need grit depends on whether you have enough smallish rocks around for them to eat. The soil here is essentially loamy sand but my driveway is crushed limestone -- which won't last any time at all in a chicken's gizzard. I have another small hanging feeder on order for grit. I expect the level will drop precipitously when I first fill it then will decline somewhat slower after that, as in 1/3 to 1/2 gone in the first two weeks then empty after six months.
RSD