It can be as long as you want! Unless your horse is not used to grass!! If they aren’t then gradually build the time up. Start with 15 minutes one day, then 45, then an hour, 2 hrs, etc..
My horses are out in a paddock 24/7, unless we have a horse show
As long as they’re used to eating grass I don’t see why they couldn’t be 24/7 or how ever long they’re out per day :)
For me it's as much time as possible, if your horse can't live 24h/24H in a field then the better solution is to spend the day outside and the night inside 🙂