When does a horse stop suckling?

In my opinion, the mare will know the time to wean him much better than we do. Many domestic horses are weaned at about 6 months. However, in the wild (when the mare decides when to wean), foals will suckle for a year or more.

It has actually been proven that foals weaned early are more likely to develop vices, like biting, rearing, stricking, kicking, etc.

So, you can wean the horse at 6 months, or you can wait until the mare decides it is time to wean.

I would do the later. The only time I think you should wean early is if your mare is losing weight dramatically, and you have ruled out other causes, like stress, change in diet, etc. If you can find no other cause for her weight loss, the foal may be taking too many nutrients, so in this case you could try some supplements, or you should wean the foal.