We have 20 birds. (9 meat so will be leaving soon) How long does a 50 lb bag of DE last--and can it go bad? I can get a 5 lb bag of FEED grade (not food grade) from out local farm store for $6 which seems high. All the sights I have found have 50 lb bags but shipping is outragious. Want to know often I would need to be ordering it.