You have a while to wait yet. My chickens never start to lay before 20 weeks, but most are around 24 weeks. If you are too excited you can get it ready now but they may start sleeping in it. I made that mistake and now I am cleaning the boxes out everyday.
Some pullets can lay as early as 16 weeks. It is rare but not totally unheard of. I'd open them up by 16 weeks and put a fake egg in there. You want them to lay their first controlled egg in there if possible. They are creatures of habit and once they get in the bad habit of laying somewhere else, it can be hard to change them. I say controlled egg because many times the first egg or first few eggs will be laid while they are on the roost or while they are wandering around in the coop or run. Sometimes they don't know what is happening with the first few eggs and they just drop them wherever they are when the urge hits, but they usually learn pretty soon. Many will lay their first egg in the nest, but some don't. A reason to open the nest boxes a little early is to find out if they are going to start sleeping in them so you can do something about it before they start laying eggs in the poop. If they start sleeping in the nest boxes, you need to make modifications either in the roosts, the nest boxes, or their habits. Another reason is to see if they scratch the nesting material out. If they do, you need to modify your nesting box, probably by increasing the height of the lip that holds the nesting material in. I'm a firm believer in the value of fake eggs in the nest. I've had a fake egg get scratched out of the nest onto the ground. A hen that had been laying in the nest laid an egg on the ground next to the fake egg. When I moved the fake egg back to the nest (and made the lip on the nest higher so it was harder to scratch the eggs and nesting material out) she went back to laying in the nest. As to when they will start laying, it can vary a lot by the individual. Out of 23 pullets, I had the first one start laying at 18 weeks. Two weeks later, 2 more started. Three weeks later, 2 more started. So at 23 weeks I had 5 pullets laying and 18 not laying. Most but not all were laying by 27 weeks. It can really vary. Good luck with yours.
My Golden Comets started at 16 weeks the last to lay did so at 26 weeks. It will depend on the bird. This is also true for how many eggs each will lay. Ridgerunner is giving you some excellent advice.
