How long is a bale of hay good for to give to your rabbit?

Depends on how it is stored and how fresh it was when you bought it. I wouldn't feed a bale after about 6 months usually. If it's stored in a cool location with very low humidity, might be good for longer. If it's somewhere damp or warm, it'll go off quickly. Always check for visible mold or the smell of mold before feeding. The hay should bend easily without breaking (so should be supple) but shouldn't be damp. If it looks and smells fresh, you're probably safe in feeding it, but nutritional content will decline with time.