Assuming an 80cf argon tank is used at a rate of 20 cfh, it will last approximately 4 hours. If the same tank is used at a lower rate, it will last correspondingly longer. For example, if the argon tank is used at a rate of 10 cfh, it will last 8 hours. Conversely, if the tank is used at a rate of 40 cfh, it will only last 2 hours.
It should be noted that these are estimates based on ideal conditions. In reality, factors such as temperature and humidity can affect the rate at which argon is used and therefore the duration of time the tank will last.