How much grit?

Hi - like Andrew4613 I've just bought my hen house and am now excitedly building the runs - one fixed to the house and one moveable. And  ???I'm beginning to buy all the bits and pieces 

How much grit does a chicken eat? I'm planning on 3 or 4 girls, and my local farm supplier has 20kg bags. That seems an awful lot of grit! Also in the interest of recycling I plan to use crushed egg shells for calcium - how fine should this be (fine like sand, or the size of porridge oats?) - and again, how much do they get through?

Thanks - I've already been feeding the odd question - lots more to come I'm sure!!

Thanks Casey76, that certainly puts things into perspective!
Great help.