Thanks. Do you know how long it will take for me to lose consciousness?
A few deep breaths in and out are all that is required to cause unconsciousness (can be up to two minutes though), with death following in around 13 minutes, although can take up to 40 minutes. An 8.8 cubic feet tank of helium should be enough to achieve this, although a bigger 14.9 cubic feet tank better. It is possible to join up two smaller tanks but that adds complication and risk of failure.
It is probably also possible to use a small tent that can be sealed. Ideally pushing out all the air whilst inside it, making sure most air is pushed out before sealing the tent, then filling it up with helium, argon or nitrogen. However, this method might require using more gas, and does not seem to be discussed in suicide literature and forums as the preferred way of performing this method.
http://lostallhope.com/suicide-methods/plastic-bag-gas