You have to decided how much c02, usually stated in "volumes" that you want in solution and then set the pressure on the regulator to the correct PSI to achieve that level of c02 based on the temperature of the liquid.
Here is a useful link:
Draft system line balancing
So typically porters are carbonated to 1.7-2.3 volumes. If you choose 2 volumes and your beer is at 45 degrees then look at the second table and see that you need a c02 pressure of 9 psi.
Setting the regulator at 30 psi will allow you to reach 2 volumes faster than setting it at 9 psi, but the goal is to turn it down before you overshoot your target. 30 psi at a reasonable temperature would give you an extremely spritzy porter so you definitely want to turn it down and 24 hours is probably a reasonable time to do so.
Ideally you want a balanced system where the serving and carbonating pressure are identical. You can read the link above for a detailed treatment of the topic but the very brief version is that if you bought an assembled beer line it is probably between 5 and 10 feet and that will probably work ok most of the time.
Just kegged a honey porter..
Many a new brewers encounter a new keg under pressure, and when they learn how to open it, proceed to bang up their hand trying to get it open (even after relieving pressure). A rubber mallet, or quick tap with palm of the hand prevents you from scraping your knuckles on the rim. It'll come in handy later when you empty this keg 
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