The average horse drinks around 30-50 litres a day. The actual amount required, depends on their size, the weather and how hard they are working.
A horse deprived of water may only live up to 3 or 6 days.
The basic rule is - a horse should always have access to clean drinking water.
If you are concerned about a horse not having access to water, arrange for a basin of water to be placed near the horse and keep it topped up. Always be careful around horses as they can be dangerous when spooked or nervous.