That's somewhat tough to call. I've been doing it for years "just whenever", probably half a dozen times a day average.

My reality check bounced last night. I had a false-awakening. I suspected it to be a false awakening (although I don't know why). So I did the pinch-nose-and-breathe thing. I was able to breath. I thought: "Why, I must be dreaming! If I'm dreaming, the wall behind my bed won't be solid" I pushed on the wall, and my fingers sank into it. I thought "that's really weird" and that's the last thing I remember from any dream in that REM cycle.