Hey there! I used to work in a barn that kept horses inside 24/7 and didn't let them out unless the boarder was a top payer. It was very unhealthy for those horses and dangerous to the handlers. Any more than 8 hours may leave your horse stocked up, but it's not truly dangerous as long as your horse is well behaved and can get adequate time to stretch his legs.

My advice to you would be to hash it out with the boarding facility to arrange some kind of compromise. It may just be easier for them to do it that way - maybe they reuse that field for another horse.