In general it's not a good idea to seperate mum and chicks at any point. Hens have been hatching and raising chicks for centuries without our (human) help.
On a more positive note, some of my best broody hens stopped mothering at 4 to 5 weeks. Much depends on the rest of the flocks reaction to having new members.
In a free ranging keeping arrangement the quicker the mum stops mothering the more likely it is she will survive. While she is still protecting the chicks in the event of a predator attack it is she who is likely to be the casualty. In general a proven broody mum is of more worth than the chicks particularly considering some of the chicks are likely to be male and for many keepers, present problems once they mature. If the hen has gone broody once it is likely she will again and chicks lost will be replaced by chicks from the next hatch.
