Hay can last for a very long time in open air as long as it is not exposed to moisture, sun and extreme heat, HOWEVER, there is something about bunnies and their hay racks and leaving perfectly good hay in there. What I usually do is just put a new little handfuls in the rack a couple of times a day and mix it around. Another trick is to just put the hay from the day before back into the main hay storage, and get a new handful of hay. There is something about the “new” hay (even though it’s not any more fresh). I think it is just more fragrant when it first comes out of the bag of hay, and then when it sits out in open air, it sort of loses that fresh fragrant scent, but it can be refreshed by putting it back and mixed.
I notice that at the rabbit rescue – even if they got their hay racks filled that morning, and you put a new handful of hay in their rack that afternoon, they’ll go after the new handful – even though it’s not any fresher than the morning hay.
Hopefully that makes sense! Let us know how the “refreshing hay” a couple of times a day goes.