For a new gun the more often the better. If you clean and lube it after you play each day it's only going to help. If you play once a month or less you should lube it every time you play because the lube will settle and get sticky as the gun sits. If you play more often then every 2-3 cases is a good idea.
DON'T FORGET TO LUBE THE POPPET WHEN YOU LUBE THE MARKER!!!!!!!!!! Its the most important part to lube and most people neglect it.
It depends on the lube you use as well. If your using a thinner lube (like the stock lube) it should be done more often. If you use a thicker lube like dow 33 or sl33k you can do it less often.