As some have mentioned, when you have a thick bed of shavings on the bottom of the brooder you can just stir it up for a couple of days before total replacement, UNLESS you notice it is getting damp in which case you need to change it sooner. I put 5" in the bottom of my older chick brooder this last time and it's been 4 days, it still looks clean and no smell since the chicks keep scratching everything around and it has stayed dry with very little poop showing on the surface. I will be changing it today though because I think 4 days is plenty long for one bunch of shavings to last.
