I've never had the displeasure of using a disposable bottle, but from what I've read, the regulators tend to be rubbish and you have to open them fully to get a reasonable flow of gas.
You can however get a gas flow measurement device, which you put over the end of the gas shroud and pull the trigger when the welder is off, and it shows you the gas flow rate:
http://www.mig-welding.co.uk/shop/gas-flow-tester.htm