I waited until all of mine were confirmed in roosting only on the roosts (at about 13-16 weeks) and were starting to show some "point of lay" signs: red combs and wattles, squatting for me when I approached and, oh yeah, my little roo jumping on them (in an amorous way!) The reason for this is that you don't want them to get in the habit of sleeping or roosting IN the nests, which makes for very messy and sometimes broken eggs. I put some fake eggs in the nests, and sure enough, this has worked well. No one sleeps in the nest (it's lower than the roosts) and they've laid their little eggs in there from the get-go.