Answer:
Some are weaned at 4 months, most at 6, and some not until a year. In the wild, a foal will nurse for about a year, and if it is a filly, possibly longer. Foals weaned early are proven to be more likely to develop vices (biting, kicking, rearing, etc.) I think the best thing to do would be let the mare decide when to wean the foal. The mare knows when the foal should be weaned much, much better than we do.
The only time i would advise early weaning is if the mare is rapidly losing weight.