I can lucid dream pretty much any night when I get a decent amount of sleep and am focused on lucid dreaming, but since I usually either stay up too late or have to wake up early, that usually works out to about three a week.

How frequent they are can vary a lot, and depends on a number of factors, probably the most significant being how experienced you are at lucid dreaming. The more practice you have at realizing you're dreaming, the more frequently you'll be able to do it.