How Common are Lucid Dreams?
This is hard to say. Many people do not understand what lucid dreaming is (either not knowing that it is possible to know you are dreaming or confusing the term with "vivid" dreaming). It has been repeated many times that most people have a lucid dream one or twice in their life, spontaneously, but wake up right after the realization (remaining in a lucid dream requires a mental composure and/or stabilization techniques because the excitement of being lucid seems to over-stimulate the mind and force either an awakening from sleep or, entry back into regular dreaming). Whether or not most people actually experience this brief encounter with lucid dreaming, I would have to say that these dreams are rare for the general population. A few people seem to be "naturals" at this thing - it is normal to expect variation between people. And although I doubt that many children have the capacity to lucid dream, some people swear that they regularly did/do so - the lucid dream prodigies. The people who start off young claim, more often than not, that they learned to lucid dream as a means to escape nightmares - rather than wake up, they learned to "wake up" inside of their dreams and handle the dream threats in creative way (usually through flying or otherwise facing the "monster" head on).
When practicing lucid dream induction, LD success tends to occur in clusters - several lucid dreams in one night or over a few nights close to each other in time. Our brains are either receptive to lucidity or not, and this depends on what's going on in our daily affairs (including how well we implement our induction regimens). I don't know if anyone actually lucid dreams every night for any appreciable length of time, but it is reasonable to expect that you can achieve one or two lucid dreams a week, or per month, without spending inordinate amounts of time and energy into the practice.