On the bag its says 5-7 days i think-but in the colder weather if you were to unwrap it it may last longer than that?- but it would be dependant on how much you use.
My 2 horses were on it for a couple of weeks when my haylage suppliers couldn't get to me safely because of the snow and bad weather- I had to open a bale every day, but I didn't mind as it was very nice stuff and they really seemed to like it.
If you mean how long will it keep? Around 7 days after opening in cooler weather. If you mean how long will it last regarding feeding, then it depends on what other forage the horse is getting. I would expect one horse to get through a bag in two days or less if that is the only forage being fed.