I've just made another test... And I think I've found a better way to test it with a little more "accuracy".
This time I put one more hose inside the garbage bag. One connected to the gas cylinder and another smaller piece to breathe from it — as if it were a straw in a cup. I also plugged my nostrils with 2 spongy ear plugs.
I filled the bag with Nitrogen, closed the regulator valve and started "breathing" from it.
I think I "swallowed"/inhaled deeply about 4 times before feeling really light headed, "excited"/"euphoric"/"confused"... and then some fear also struck me — because I was sitting on a chair and didn't want to risk falling on the floor...
Breathing through the hose is quite demanding and unnatural... And I might have inhaled oxygen in between breaths, when putting the hose back into my mouth after exhaling...
After the 4th breath... I just couldn't do It anymore from that point on... so I dropped the hose and started breathing normally... I felt that I was about to pass out and fought against It.
All of this took place in about 35 seconds(I've recorded a vídeo to know the timings). Not the best scenario as I expected, but also not bad at all.
I'm estimating that, with the bag over my head, things will be a lot easier, efficient and comfortable... And with no risk of inhaling oxygen.