Arnold suggested hitting arms twice a week for beginners and 3 times a week once you were advanced. I definitely trust him more than I trust any random nikka on Coli. But I'm not sure that type of intensity is possible for everyone, especially if you are natural. Personally, I physically can't hit arms 3 times a week....not even close.
But I guess the best answer is hit it as many times as your body will allow you to without compromising the rest of your workouts. If you can hit em 2 or 3 times a week without hurting yourself or neglecting another body part then do that shyt.