A flow regulator can easily be set to 15 liters/ min. The gas will flow at this rate until the cylinder’s contents are exhausted (~300/15 or ~20 minutes which is much more than is needed for a reliable and peaceful death).
The Gas Source (Nitrogen, Argon & Helium) 1. Nitrogen
Nitrogen is a very common gas, making up ~ 80% of the air we breath. The gas is cheap, and readily available. Widely used in homebrewing, Nitrogen cylinders can be purchased (as opposed to hired), and stored indefinitely in the cupboard at home for possible future use.