I usually run around 20-25 but your nozzle/lead angle/work angle may dictate a different setting for you. If you are unsure then start at 20 and do some stringers. If it's not sizzling like bacon or you have an erratic arc then try increasing the gas to 25. More is not better. With too high of a flow rate you can actually start drawing atmosphere into the pocket due to the Venturi effect and it will contaminate your welds.
As far as how long it will last, I used to know how to calculate it, but I haven't had to do it and have forgotten the formula. If I'm not mistaken 40ft