Are you trying to force carbonate it at 30psi? If so you will need to leave it on. Shaking it or bouncing the keg will get it to carb almost instantly if your beer is cold. Results are not always good doing that though. 12psi with it cold, sitting still for about 2 weeks works better.
Somewhere around 12 is good to dispense but you will have to see how your system works. Most of us leave the gas on all of the time to dispense. Down side is if there is a leak you will find your tank empty and beer flat. Always check for leaks with a spray bottle of soapy water or Star San.