Ought to flow only when the trigger is pushed. There are a couple of types of valve though - a lot of the cheaper welders have the valve in the torch so you can squirt a little gas out by pressing the trigger half down. Expensive welders all have an electric solenoid that operates when the welder starts, so you have to squeeze a little wire out to purge the torch or air before you then chop the wire off before you start welding.
Thx for the response, haven't got that far yet. I'm deciding what size bottle to get. If the gas flowed the entire time it would affect my decision. If it only uses the gas when your welding (trigger pulled) then I don't need that large of a bottle.