Watering off horses as a point of reference depends on the amount of activity that the horse has been engaged in. For instance if you have been riding your horse in a speed activity like making barrel runs, roping, reining or jumping you should readily assess the respiratory rate amount of heat coming from your horse and the relative amount of sweat and dehydration that has occurred. The temperature of the environment and amount of humidity should also be considered. Your horse should be watered off with water that is NOT extremely cold. A large amount of cold water taken in too rapidly by an overheated horse can be a shock to the GI tract and can shut down motility and cause acute colic symptoms which could escalate quite rapidly into an emergency situation. I recommend that water be offered intermittently throughout the course of activity. You might find that your horse will perform better and be less stressed during workouts and performances than doing without throughout the course of a exercise session. I also will fill a bucket of water up near the arena that I am riding in and allow that water to warm up to avoid being a shock to my horses system.