Let them have it at all times, either mixed into the feed (which I do) or in a little bowl. Most people offer it on the side at all times from what I have read. That way the hen won't overconsume the layer feed just if she needs a calcium boost.
I go by how my shells are. If I'm getting thin shells, I just toss a handful in the run. It gives them something to do, scratching around for them. I don't have many birds laying now, and the ones that are are young layers, so I'm not offering any. As spring advances. I'll offer more as the older girls start laying again.
