How long can a horses survive eating snow?

Well, snow is not really food. It's frozen water.

If the horse is eating food but having snow as their water, You will have to give it snow constantly, like, all day! because horses drink about 10 gallons of water a day, and that snow isn't going to be enough to keep them properly hydrated. So, it's best to give them regular water.

If you're talking about the horse having snow as their food and water! The horse isn't going to get the nutrition it needs, so, It will prob only live for about 10 days or more. I know it's over a few days though.