I imagine that is different for each individual. I started attempting to lucid dream about a month and a half ago. I've had seven LDs so far. I think I started noticing weird stuff within dreams a little more consistently a week or so ago. I started cataloging my dream signs and noticed that most of them are Context dreamsigns. So I started paying attention to context in waking life throughout every day and trying to be mindful and aware of where I am, what I'm doing, how I got there, where was I before, etc., and that seemed to transfer over to some of my dreams after a few weeks. I had a lucid within two weeks, though, by noticing my shirt was gone in a dream. I had another one within a week of that. Then this past week I had four LDs within seven days. This is all anecdotal, of course. Some people will start having LDs faster or slower. It also is probably, to some degree, contingent upon how much effort and energy you put into it.