Putting a plastic bag over your head HURTS. The power of the human will is astonishing.
It takes almost a minute to start draining the contents of the bag of oxygen (depending on the size of the bag). You start breathing deeply nearly immediately. I'm impressed at how keen the body is at keeping itself alive at all costs. After that, your breaths start becoming rapid and deep, your lungs searching for the oxygen that no longer exists.
If you don't have a large bag, which I'd recommend using actually, you suck in all of the available gas within the bag and are left with nothing but a wall of plastic around your lips. If you're unfortunate enough to experience that feeling, your lungs will go into a burning frenzy and pull as hard as possible at the plastic to squeeze one more bit of gas out in the hopes that you'll find a hidden goldmine of oxygen somewhere. If the bag is large enough to support the full extent of your lungs' capacity of CO2, the process will continue for another agonizing ~30 seconds. After that point, your brain, low on fuel itself, will begin tearing into a frenzy trying to pull oxygen for itself, depriving the rest of your body with the energy to continue on. You'll painfully find yourself getting duller an duller, all the while you diaphragm screaming in pain. If you have the fortitude to continue... I cannot tell you what awaits. This is the part of the journey I got off at.
If this is what you seek, I wish you the best of luck and the hardiest of wills. I envy those with the courage to follow this path to the end.