That may depend on strain. Assuming they are like mine, then you have 30 to 90 days before separation becomes mandatory.How long can i leave these guys together in their little pen?
I look closely at tail feathers. Feathers of greatest interest are the two sickle feathers at top. When chicken matures from hatch to adulthood it goes through three feather sets. First is usually brown with extensive barring and tail feathers begin to be replaced by about 7 weeks. The next set are darker and on tail can be used to tell sexes apart. It takes about 6 weeks for those feathers to mature. When those sickles are mature, they look like those on your stags. They will drop soon and be replaced by longer shinier version which will be retained in first adult feather-set through molt next fall. As those feathers come in and near setting, hormone changes are going to start making so stags will be rough on each other. My recommendation is to separate before those new sickles feathers are fully in.
I also like to have stags running free through that time. I am of the opinion that free-ranging until most mature makes for a healthier adult.