this may sound like a dumb question, but when do you know how much gas to allow while welding? like, what are the circumstances to having too much or not enough gas in the weld?

 

The consequence of too little gas flow is poor gas coverage resulting in porosity in the weld.

The consequence of too much gas flow is about the same due to the high turbulence of the gas allowing atmosphere to mix with the shielding gas.

Generally you would use 10-15cfh for most short circuit mig welding. The same flow for most tig welding and 25-30 cfh for spray arc mig welding or dual shield.

If you are tig welding you can run higher flow rates if you are using a gas lens due to the lens diffusing the gas better at higher flow rates.