Co2 Question?

I built my keezer today and I have my first corny in it. My question is with the Co2. I know when I force cold carbonate I need 15-18 psi to start with. Do I keep the Co2 on or when it reaches the 15 psi I turn the tank off? I know dispensing pressure is 5 psi. I also know I need to pressurize to 15 psi and then shake it, then repressurize. Do that a couple times a day. Between do I cut the Co2 tank off?

 

I follow Yooper's advice: set the pressure to 12 at 37 F, and then forget for a couple of weeks. Leave the tank on.