You could read the pressure any number of ways. If you already have a boost gauge mounted, it should read the correct pressure (it should be on, if its electrical). If you used a valve stem, you could use a tire pressure gauge.

Ideally, the system would maintain 15psi until it you released it. Invariably, though, you'll lose some pressure through the valve seals and rings. I would say that you should be able to pressurize the system, disconnect the comressor (or whatever) and have plenty of time to listen for leaks before you have to pump it up again.

Oh, make sure you're not trying this with one of those valve cover breather things!