That did sound a bit confusing didn't it? Basically what I was getting at was that if I increase the pressure to the tank above 30psi, the hose will come off where I have it attached to the airline adjuster. That make sence?
If the regulator has 2 dials on is, and has a fine control knob, then yes. You need to make sure though that the output dial to you tank is capable of having a low psi on it. With regulators with dials that go up to 500psi or more it will be very difficult to control low psi's.
You will know when the cylinder is getting low when the dial telling you the pressure for it will drop from 800psi. When there are no more bubbles in your bubble counther, the cylinder is empty and you need to re fill it. When it is empty, you need to refill as soon as possible, because the ph will hike back up. I don't like to leave it for more than a day. I generally go out as soon as I see that it's empty. If you don't have a car, or rely on someone to get you places, this could be tricky. Also, check at the place you buy the tank as to how they re fill them. Some places send them out for filling and could take up to a week to get back to you.
It's impossible to say how long a tank will last. It all depends on the size of your aquarium, the size of your co2 tank, if you turn it off at night/use a solenoid, and what pressure you have it at. I have a 5 pound tank, and people here were telling me that it should last me like 6 months. I have been getting about a month and a half on mine.