How long do iv fluids stay in your body?

Depends: this actually depends on your hydration level. your kidney maintains the homeostasis of your fluid balance. if you are dehydrated, they would work to keep the fluids in (anti-diuretic).

Depends.: No longer than any other fluid. Body moves a liter or two of fluid each day through the system. If you over fill the system by one liter, kidneys will promptly excrete it. If you are a liter low your body will hang on to any fluid and produce only minimal urine until the volume is back to normal.