system
Given the following post, respond as if you're commenting on Reddit:
human
How long can I reuse the same breath?

So I have a balloon like device attached over my mouth, it easily inflates when I exhale.

How long can I continue to use this device (and therefore the same volume of air) before I blackout/die?
gpt
Is there any air in the device to start with? 

Are you pre-breathing 100% 02 or room air?

 If it starts with 0 volume and you're an average male, that means you'll exhale ~600mL of air, since we use about 25% of the oxygen we inhale on a single breath you will have about 95mL of oxygen exhaled into the bag.  We normally use 250mL/minute of oxygen, you won't be able to extract all 95mL of oxygen because of diffusion pressures anyways, so you will run out of oxygen in under 30 seconds.  Passing out can vary wildly, some individuals can last minutes, others will last 15 seconds.  Syncope is not something we have a defined limit on for oxygenation at least, but I'd go somewhere within 60 seconds for most persons.

If you were pre-breathing 100% 02 that means you'd have 600mL of oxygen to work with.  The problem here is that you exhale CO2, and it will again screw with the diffusion pressures.  Let's work within the 250ml/min and an average rate of 12 breathes per minute.  That means we're extracting ~21mL of 02 and exhaling close enough to that amount of CO2.  That means in 1 minute you'll be at 350mL of oxygen, and 250mL of CO2, this will drastically hinder oxygen uptake within the lungs, the pp of the CO2 within your blood will actually prevent it from diffusing back across, making you acidotic and reducing the Hgb affinity for oxygen further.  Thus, you'll be unable to clear CO2, and become acidotic, this alone won't make us pass out however, until CO2 becomes high enough to make a person faint (again, highly variable, let's go with 140mmHg as a reasonable guesstimate point however.)

This of course all presumes you have the nose perfectly sealed the entire time.

If there was a way to evacute just the CO2, you'd have considerably longer, but in the case of being unable to evacute it, we're in the same neighbourhood as the original, somewhere right close to 60 seconds.