Keep waiting! Keep the Dream Journal, keep reading about lucid dreams, drink plent of dairy (if you havn't not an allergy) and you'll soon get it. Unless you want to try any methods.

To answer your question I think it took about two - nine months for me to get my first lucid dream since joining the site.

There's not really an avarage to measure really, unless someone does a hardcore survey, which I might try and pull off myself.