The first two parts to this are easily identified. You can read on the shoulder of the tank its fixed volume and you can read from the pressure gauge how much it has been filled. Multiply one by the other. A 10 litre tank filled to 200 bar has 2000 litres of air in it. A 15 litre cylinder filled to 230 bar has 3450 litres of air.
It is best to set aside a reserve of air and conventional thinking suggests that quarter of the initial supply is kept aside. This may be over cautious with a large tank but you have to make a judgement based on the circumstances you expect to encounter. Let's assume that our 10 litre tank has only 1500 litres of air at our disposal with the rest (50 bar) is held in reserve.
The next thing to identify is the depth the air is going to be breathed at. At 30m deep, the regulator delivers air at four times the pressure as it would at the surface. Thus if we are diving at this depth, we have only 375 litres of air to breathe.
The final part of the equation has a big question mark hanging over it. How much air do you need?
A woman with small lungs will probably breathe a lot less than a heavy-weight boxer. A man with large lungs will have the ability to pump a huge amount of air through his lungs. A relaxed man might breathe only eight litre every minute but increase his heart-rate by increasing his work load or stress him in some way, and this can leap to 30 litres each minute. It has not too much to do with fitness either. An old diver who has smoked all his life may not be very fit at all but if he is relaxed, and often that comes with experience, he will use less gas than a young trained athlete who is working harder than he should underwater. Even thinking hard uses a lot of energy. If you have to swim hard you will consume more than if you are merely hovering in the water.
So what figure should we use for a breathing-rate in this calculation? Training agency manuals usually use a figure of 25 litres per minute in their examples of how to calculate air consumption. At 30m, our 10 litre tank (with 50 bar held in reserve) would last only 15 minutes.
So what does it feel like to breathe from a regulator underwater? You can easily try this for yourself because it feels exactly the same as it does if you try it on land in a dive shop. You suck, there is a faint resistance and the valve pulls open. The mouthpiece floods with air that you inhale. It stops when you stop. When you exhale, there is also a slight resistance as the exhaust valve opens and allows that air to escape. It bubbles away past your head.
Remember, the amount of air in your tank is divided by your respiratory mean volume or breathing-rate multiplied by the absolute pressure in bars at the depth at which it is breathed.