system
Given the following post, respond as if you're commenting on Reddit:
human
How long does it take for oxygen in your body to cycle?

You breathe in oxygen, it gets attached to hemoglobin, which delivers it to cells where it gets used by cells to generate ATP, producing carbon dioxide, which gets pumped out of the blood by that same hemoglobin and dumped back in the lungs. How long does this cycle take? E.g. if you were to breathe in some oxygen 17, how long would it take for it to exit your body?

Similar question in regards to water that you drink - how long does that cycle take from absorption to excretion?
gpt
Maybe just a minute or two, but also maybe hours, years, or never. 

A rule of thumb is that it takes your blood about one minute to make the full circuit of your body. Oxygen and carbon dioxide diffuse very quickly in and out of cells, and the reaction that makes ATP is happening all the time. So if a given oxygen atom is involved in that reaction and becomes part of CO2, it will soon wind up back in the bloodstream and may be breathed out.

Not all CO2 is eliminated from the blood in a single pass through the lungs. Some always remains in the bloodstream. So, odds are that your tagged CO2 molecule will need to take a couple of laps before it gets exhaled. Call that a matter of a few minutes. 

However, not all oxygen you breathe in is destined to become CO2. 

For example, some of it will oxidize hydrogen, making water. That becomes part of the water supply of your body. You do breathe out water vapor, but more of it sticks around longer and gets eventually eliminated as urine. A tagged water molecule would circulate for at least a matter of hours before having much chance to be excreted, and for a very high chance you'd have to wait days. 

That's not all. There are also oxygen molecules in bone. That's not a major use of the oxygen you breathe in, but bone is being remodeled and rebuilt all the time, and there's always some chance that your tagged oxygen atom will be used in that process. If that happens, it will become a permanent part of the bone, and might still be there long after your death.