I think it should be lots. I'm more familiar with hoods, so I'll discuss that.
It's important to flatten the hood removing atmospheric air, and letting the hood "inflate" before you place it over your head. This creates a reservoir of fairly pure inert gas that you can draw from, instead of breathing strictly from the gas hose. Assuming you create this reservoir and exhale maximally (so that you dont exhale a bunch of CO2 into the bag) before placing if over your head, you will only need 15 liters per minute of continued flow to maintain 100% nitrogen concentration in the bag.
A Dignitas study on helium inert gas asphyxiation (
see link here for study) found that at the very most, death should take around 40 minutes. However, in most cases it only took 5-10 and the outliers were suspected to have leakage of oxygen into the breathing apparatus, sustaining life for an extended period of time. With 400L of nitrogen gas, you will have 26.6̅ minutes of full flow.
The question I ask in return is this: how confident are you in your apparatus? If you are confident it will not leak, the amount of nitrogen should last over double the required time for you to die. However, if it leaks, there is a chance for complication.
Another factor to consider is that assuming you have a good seal and you exhaled maximally prior to placing the pre-filled apparatus over yourself, you may not necessarily need continued flow beyond ~30 minutes. At this point, you should definitely be in respiratory arrest (if still alive at all). You will not be breathing, and even if you were, the apparatus should be sealed and have a minimal amount of CO2 and O2 in it. The inert gas will essentially just cycle without reacting with anything or becoming dispersed, because it is
inert and
sealed in the apparatus. I saw a report once of someone who committed suicide with one of those tiny Coleman camping propane tanks, because once it filled their exit bag it essentially just stayed there and asphyxiated them.
I think with most suicide methods, the phrase "a chain is only as strong as the weakest link" applies. With SN, you can take the lethal dose but end up vomiting it up. That's why most people who use SN attempt to get an antiemetic to reduce risk of throwing up the SN. For inert gas asphyxiation, I think that for most people the weakest link is the breathing apparatus, and not the gas supply. If you have a proper seal, 400L of nitrogen is more than enough. You could probably get away with 200L if you had a perfect seal, but this isn't ideal because it leaves little room for error and its hard to "test" a seal. Whether you use a hood or a mask, my advice is to make damn sure that it doesn't have leaks, and that everything is secure. I've seen threads of people discussing the gas hose falling out of place during a failed attempt, them knocking over the gas cylinder while unconscious, or the bag/mask itself falling off or becoming unsecured.
I don't have access to nitrogen, so I'm considering propane asphyxiation because it essentially functions the same way (lmk if you want more info). I plan on using one of those big oven roasting bags, securing the hose to the inside, and then duct taping the bag snugly around my neck so that it stay in place.
