you use it 90% of the time, every car is different. Normally I keep it closed on initial startup for about 30secs then open it up to make some heat and drive around in the pits. It will not raise the rpm like closing a barrel valve does. You can buy all the stuff you need from Mcmaster car, they sell a slick slide valve used for oxygen tanks, fittings and hose. Install the valve somewhere in the cockpit that you can reach while strapped in.
IMHO, 1/8" fitting is too small at least 1/4 or 3/8 to allow enough air in. Then you will save some fuel...