It took me about a month, which seems to be about the norm. It certainly varies from person to person, but many usually fall within the 1-2 month time period. The more effort you put into it, and the less bad habits you develop, the faster you'll become lucid.

I prefer the nose pinch RC. Pinch your nose shut and try to breathe through it. Also, the clock test works best with a digital clock, rather than an analog one. The rule of thumb is to reality check as often as you need to in order to become lucid. Some people rarely need to RC, some do it as frequently as every 10 minutes or so. It varies from person to person. The main goal of RCing is to boost awareness, too, so that when you recognize something unusual, you can identify it as such and check reality (RC) to determine if you're dreaming or not.