While Parkerizing has been around a lot longer than many of the newer, more modern firearms finishes, a lot of younger shooters might not understand how it "works" so to speak. Parkerizing is different than most all of the newer, more modern firearms finishes on the market today, in that it offers very little protection on it's own.
Parkerizing itself is only a media for holding oil near the steel itself. Thereby allowing the oil to protect the firearm steel from rust. Parkerizing left dry and free from oil offers almost no protection in the way of rustproofing. It requires oil to be effective. The question then becomes how much, and how often?
This is a matter of personal preference and location. Also if the gun is carried in a leather holster or not. If the gun is exposed to high levels of rain or humidity, or stored in a leather holster, it will require more and heavier oiling than if it's contained in a Nylon type of holster in a dry, desert climate.
Through the years I have found the best way to initially treat Parkerizing, is to oil it very heavily, and allow it to "soak in" overnight. Then, with a absorbent cloth, or a paper towel, blot up as much of the excess oil as possible. Don't wipe or scrub the surface because it will transfer lint like crazy, because of the Parkerizing's naturally rough surface. If you live in a humid climate you will want to repeat this process often to be sure the surface of the Parkerizing itself has oil contained in it. Oil saturated Parkerizing has a darker color to it. Dry Parkerizing has a more greyish cast. Below is a picture of my Springfield Loaded Parkerized 1911 pistol. You can clearly see the surface is wet with oil. A Parkerized firearm in this condition will all but never rust.
Parkerizing requires more maintenance, but properly cared for it will serve it's purpose well.