Hay can stay good for up to a year when properly stored in the ways stated above. You can tell when hay is not good when it has a musty smell and is extremely dusty. Hay can be a bit dusty, but when it is bad, it looks like huge poofs of clouds when touched.
I asked my vet one time, "so actually HOW much is a rabbit SUPPOSED to eat per day?". I wanted an exact amount per body weight. I was frustrated because everywhere I researched, I could never get a particular amount for body weight. She told me the reason why: It all depends on the greens and pellets they get, and how much fiber is in those. HOWEVER, on average a rabbit should consume about a full handful or two of hay per day. And when I say handful, I don’t mean what will just fit in the palm of your hand – but as much as you can grab with one hand. However, my bunnies do have unlimited so they can eat more if they want. Jack probably eats 4 full handfuls of it a day, while Bailey may eat one and half, and Rucy, well she’s lacking in a few back molars, so I actually have to push her to eat hay.
Since hay can stay good for much longer than a month, just grab a few bags, and soon you’ll figure out what your bunny actually consumes.