My own foals have also been returned to their herd after weaning, but have been separated from their mother for weaning to ensure that the mare dries up as fast as possible. If a foal is being kept by the breeder then obviously living with the herd is the better option, but as the original question was when can a foal leave it's mother, it can be assumed that the foal will not be returned to it's herd.

A foal is usually weaned from it's mother at 6 months or older. Most people wean by 8 months. If you leave a foal feeding from a mare too long it can lead to the mares condition deteriorating, and to separation issues when you do want to wean. Some people will leave a mare to wean a foal naturally if the mare is back in foal again, because a mare will naturally want to keep her energies for growing her new foal.