As you start seeing signs they are approaching the point of lay (reddening of face/comb/wattles, submissive squat, etc) or even when you find that first egg are good times to open boxes previously kept closed.
I opened mine up at around 15 weeks. Put 2 golf balls in each box. None of my 3 showed much interest in the boxes until they were ready to lay. First one was laid from the roost! She put the next one in the nesting box!