It depends on how long it has been since my last shampoo. Or how hot/sweaty I've gotten in the last day or so. If I'm showering every other day on schedule, I was my hair then - just one soaping. Otherwise, I will do it twice if hair/scalp are very dirty.
Like most people said, I only repeat if I'm really, really dirty or if I used a lot of product in my hair for some reason. I have curly hair, and I can't see never shampooing. I heard you're not supposed to, but any time I don't, my hair gets very greasy and I feel like I can just scrape oil off it. And I have DRY hair! I can't stand touching it if it hasn't been shampooed.