The rule of thumb is 20% of its bodyweight, maximum. A horse's spine is a single piece from between its ears to the tip of its tail, so having it carry the proper weight is important, especially for sport. I don't know what breed OP's Duffy is, but your average riding horse is between 1,000 and 1,300 pounds. So 200-260 pounds, though most barns I've ridden at cap riders at 250 pounds, just to have a hard limit set.