There's really no set time when they'll start laying. My first batch of LF breeds (buff orp, barred rock and EE) all started laying at about 4 months. I was diligent in closing the boxes at night so nobody slept and pooped in there, but removed the blockade after bedtime so they were open first thing in the morning -just in case. I caught them quite frequently visiting the boxes during the day and rearranging the fluff and fake eggs... instincts were telling them something about this was important.
You don't need to stop everything and build the complete nest boxes, you can use temporary substitutes, like 5 gallon buckets or plastic totes on their side, milk crates, wood crates, large baskets, a covered cat litter box etc. Just set them on the ground where you plan to build the final ones, and fill with excelsior nesting pads or a thick layer of hay or straw with some fake eggs or golf balls to encourage them. Some people keep a variety of different style boxes at different heights because not all birds prefer the same kind.
If you don't have nest boxes available, they will find somewhere else to lay when they're ready and you might not get them to actually use the boxes later on. If they free range, you might not even find the eggs.
