If you are using the rug to sweat off the horse, it should stay on the animal until it stops sweating. A good sweat rug should allow the horse's sweat to disperse to the surface and appear as small droplets on the top of the rug. The time needed for the sweating to stop will vary from horse to horse. It also depends on the intensity of the exercise and the thickness of the horse's coat. You therefore need to keep checking by putting your hand between the rug and the horse's back.
If you want to use the rug to reduce the growth of the horse's winter coat, the rug should be left on the horse during both day and night from October onwards. Of course, this does not apply when the horse is working. This way it will prevent your horse from getting cold while suppressing the growth of the winter coat. For this purpose, the rug should not only have good chest fastening, but also cross straps on the belly.