Leo Babauta of ZenHabits.com recently writes in his post The Habit Change Cheatsheet: 29 Ways to Successfully Ingrain a Behavior:

3. Do a 30-day Challenge. In my experience, it takes about 30 days to change a habit, if you’re focused and consistent. This is a round number and will vary from person to person and habit [...]

There is almost no way that you haven’t seen them. Those mildly annoying commercials featuring the "Video Professor", touting his "educational" wares (they are on seemingly most TV channels dozens of times a day).
And while he’s been around for years, lately his ad, centering on a "How to Sell on eBay" course CD that [...]