How long after a foal leaves the mare before milk dries up?

Most mares will dry off within several weeks to a month. Some never dry completely. Some that are apparently dry will produce milk again if the foal goes back to nursing. As a general rule keeping the foal from the mare (and if she's a mare that will nurse anything, keeping her away from other foals) for several months is enough. I have however seen 2 and 3 year old foals go back to nursing their mothers. In addition, if the foal is a colt and is more than 6 months old he should be gelded before returning to his mother after weaning as they can be fertile as youngs as 7-8 months of age and have no qualms about who they breed.