You can get all kinds of conflicting advice on this forum so I'll add to the confusion. I want the nests available about a week before they start to lay. That way if there are problems I can fix those problems before I get poopy eggs and I have a better chance of them using the nests instead of learning to lay somewhere else.
When are they going to lay. There are many things people will tell you that are clues they might possibly be ready to lay, but that's all they are, clues they might be ready. I don't rely on those clues but go by age. The earliest I've had a pullet start laying is 16 weeks so that is my absolute deadline to have the nests open. A little earlier will not hurt.
What kinds of problems am I talking about? Often a few days before she starts to lay a pullet starts looking for a safe place to lay her eggs. Often, not always. Egg laying is fairly complicated and sometimes it takes a pullet a few days to get all the kinks out of her system. Some seem to have no control over the egg-laying process and just drop their eggs anywhere. But once they gain control they want a nest. Checking out a place to lay usually involves a lot of scratching. If you find your nest bedding or fake eggs on the coop floor it usually means your nests are not right. You probably need to raise the lip to make it harder for them to scratch stuff out. I prefer to not have real eggs scratched out of the nest and on the coop floor. A lot of the time you can head this off by having the nests open early.
The big reason I see people give for blocking the nests is to keep them from sleeping in the nests. If your roosts are higher than the nests this is hardly ever a problem. It still can be. If your roosts are inadequate in space or you have a roost bully (which happens) one chicken may pick on a lower-ranked chicken so brutally they seek a safer place to sleep. That could easily be your nests. This happens a lot more often when you already have mature chickens and you are adding new chickens, but it can happen with them all the same age.
If your chickens are sleeping in the nests there are reasons for that you need to fix. Even if you keep the nests blocked until you see eggs they can still move to the nests to sleep when you open them. I prefer to fix those things before I get poopy eggs or eggs scratched onto the coop floor.
There is nothing wrong with waiting to install the nests or blocking them off, but I want them available before they start. And as JT mentioned, once you have hens laying the nests are going to be open when you raise your next batch of chicks.