The number's going to vary drastically from person to person, but about three a week seems average for most. Personally, my current average is about two or three lucid dreams a week. I could probably do twice as well if I had more control of my sleep cycle, but unfortunately I have to wake up early most mornings. I don't need to devote much wake time to lucid dreaming, just a few seconds to do a reality check every once in a while.