Chewing up the cuttlebone and/or mineral block can be a sign she is thinking about going to nest, there is no general rule as to how many days before she will start laying, I have had a hen totally tear into a cuttle bone just as fast as I could put them in the cage, but has never laid an egg. You will know within a couple days when she is ready to lay, her back will develope a slight hump in it, her tail will bob more, her poop will become more liquified, when you see this, it is probably a good sign she will lay soon, BUT, even that is not a for sure thing, only a guidleline. Also, as a rule of thumb, a hens cere, will turn a dark chocholate brown when she is in condition, also, not always the case, I have 3 hens right now, that do not have any of the signs that I just mentioned, but all have 4-8 fertile eggs! (although they did get a slight hump in the back before they layed, it is kind of a saddle for the cock) Hope this helps, good luck, keep us posted!
Dan
www.bluwillowfarm.com