Rodgervich:
Try posting this question at the forum for pilkguns.com It's a serious airgun site for competition shooters.
I use a PCP target pistol and charge it with what looks like a fancy bicycle air pump. This type of pump, in addition to attaining the necessary pressure, also traps moisture that could corrode the gun's air cylinder. For my pistol, it takes about two minutes of pumping to charge it at an easy pace. A rifle will probably take 3 times as long, but as someone who knows. BUTCHER45 on this forum hunts with a PCP and should be able to tell you.
A PCP is (thankfully) in no way similar to the CO2 powerlet guns. Mine is a match gun of very good quality. It holds a charge and loses only a little pressure after sitting unused for the summer months during which I do not use it. I cannot vouch for guns of different quality levels. Maybe some of the lower end models leak more, but I seriously doubt that they would ever leak as much as a CO2 gun.
I top mine off every 70 shots or so. It will shoot about 250 if I shot from full to empty. Number of shots varies greatly depending on the model of gun and velocity.
PCP guns are also not as susceptible to shot variation due to temperature changes, as are CO2 guns.
One important thing I've observed about PCP guns is that they are surprisingly loud-- both pistols and rifles. So if you're shooting indoors, you'll need ear protection.
I cannot tell you much about the SCUBA tanks except that if you live in a humid climate, you should use the SCUBA tank instead of the pump. This is because SCUBA air is dried, and that will prevent water and subsequent corrosion of the gun's cylinder. I live in a place with low humidity during Winter when I shoot, so the hand pump is a good option.
