I don't know if the DE expires or not. I'm going to say that as long as its not wet then it should be fine. I've had an open half bag in my shed for over a year and I still use it occasionally if its closer to me, lol. If you do have to get more, I would get a storage container to put it in so it doesn't get wet or anything. Not like you have to worry about bugs getting into it! lmao
What I usually do is; check the weather forecast to make sure its not going to rain for a few days, then I dust the entire plant with the DE. If it does rain then just re-dust. I usually dust during the spring, summer and beginning of fall every two weeks or so. I just put it in a plastic cup and walk around with it. One solo up will cover, pretty much every fruit bearing or flowering plant I have.
Like I said, its completely safe for animals; so if you were to track it in it would be okay. I dust my actual chickens and their whole coop/house.