100 g of baking soda + 200 g of citric acid should be enough for producing effective concentration of CO2 inside a 20 L or smaller bag quickly. Alternatively, it's possible to use a soda siphon with 8g CO2 chargers to inflate the bag - this would be an easier way actually.
I'd rather carry out the reaction in a plastic bottle with a plastic bag attached to the bottle's neck. Once the bag is inflated enough, it can be placed over the head and sealed around the neck with a PVC electrical tape or something like that while holding breath. When sealing is done, you resume breathing and just wait for LOC, trying not to rip off the bag.
Within half a minute. Maybe a bit more if you're unlucky.
A failed attempt may result in complications with lungs, brain, heart, and possibly other organs.
In a sealed environment, you'll be exposed to CO2 all the time. At high concentrations, this gas induces narcosis, even when it's mixed with a lot of oxygen, such as 30% CO2 + 70% O2. Waking up from such a narcosis without stopping yourself to be exposed to critical levels of CO2 further is very unlikely. Besides, CO2 produces respiratory arrest, which may occur in 1 - 1.5 minutes according to some sources. After cessation of breathing, the concentration of CO2 around your head won't matter much.
I recommend to read
https://i.sanctioned-suicide.net/images/2025/03/224148_NIOSH_-_CO2.pdf starting from page 25 (Effects on Humans).
30000 ppm is just a STEL (Short-Term Exposure Limit) value
The revised IDLH for carbon dioxide is 40,000 ppm based on acute inhalation toxicity data in humans
www.cdc.gov
Short-Term Exposure Limit (STEL) refers to the acceptable concentration of a hazardous substance in workplace air for a short period, typically 15 minutes. This limit is designed to prevent acute health effects that might arise from short-term exposure to high levels of a toxic substance. STEL is especially relevant for substances that can cause immediate health effects, such as irritation, dizziness, or respiratory issues, after a brief exposure.
It would take eternity to lose consciousness and die from this concentration of CO2, compared to plain plastic bag suffocation that could render LOC within 10 - 15 minutes. The whole point of CTB via acute CO2 poisoning is that this gas can produce unconsciousness in half a minute, which is a relatively short time, making the method worthwhile for those people who don't mind to perceive some possibly severe discomfort as long as its duration is short and death is almost certain. To gain this efficiency, you need to add one more "0" in 30000.