I displace the volume of the system by a factor of 6. That usually is sufficient for stainless. However, often overlooked by the welder is the need to clean and swab the inside of the pipe for a distance of 2 to 6 inches from the joint to remove oils, dirt, and other debris that can off gas when heated during the welding operation.

Calculate the volume of the region to be purged. That volume can reduced by employing inflatable dams to reduce the inert gas needed and to reduce the purge time. So, calculate the area (3.1416 x (D x D/4))/(12 x 12) of the pipe and multiply that by the length of pipe being purged. The divisor 12 x 12 is to give you square feet. Multiply by the length in inches / 12 to provide you with the volume in cubic feet. Now multiply that volume by 6. Divide the factored volume by the flow rate used, and that will tell you the time required to purge the system. That assumes the joints are taped (no leakage) and the ends of the pipe are sealed leaving only a small vent hole at the highest location.

Al