When their combs are big and red which might be nearing 20 weeks or might be six months depending on the breed and climate/season that they start getting old enough. If you want to switch to layer. Feeding chick feed or all flock all the time also works well if they have a calcium source (usually oyster shell and/or eggshells) in a separate dispenser.
You're going to get all different answers. I don't pull out layer until half the new pullets begin laying. There's already oyster shell and eggshell available anyhow.
This.
Team01, because many ways work well. There is a lot of margin between harm from too little and harm from too much and either takes a long time to show noticeable effects and even then is quite reversible.