How long does a bag of iv fluid last in your body?

As long as you need: Our bodies are in a constant state of balance. We keep what we need and get rid of what we don't need. This goes for water (fluid), too. If you're dehydrated & receive just 1 bag of fluids that isn't enough to replenish your stores, you'll hang on to that fluid more so than if you got a bag just for kicks (this should never happen when not dehydrated) in which case you'll pee it out within hours.