How many days does it really take to form a habit?

The traditional duration is 30 days. Here's a good example by Steve Pavlina:

A powerful personal growth tool is the 30-day trial. This is a concept I borrowed from the shareware industry, where you can download a trial version of a piece of software and try it out risk-free for 30 days before you’re required to buy the full version.

Celestine prefers 21 days:

1. 21 days is what it takes to fully break/form a new habit 2. A 21-day trial is more efficient than a 30-day trial

Dragos Roua thinks 15 days is enough and he has developed a day-by-day system to prove it.

Three comments:

1. Belief

How fast a habit is formed has a lot to do with belief. If you believe that you can change your habit in a week then you can change it in a week; if you believe it will take a month it will take a month. Celestine twits:

Argue for your limitations and sure enough, they're yours...

2. Emotions

The number of repetition (days) is not the strongest factor forming the habit. I believe that the strongest factor is emotion. A split second may be enough to start a habit. Lovers' first date is the start of a habit. The habit is formed by pure emotion and without repetition.

3. Environment

Another factor is the environment. If I'm starting a transition to raw diet by following a habit formation plan for 30 days, I may reach my goal in 2 weeks if I am living in a tent with no kitchen in a Mediterranean island with plenty of delicious raw vegetables and fruit available in abundance cheaply. If I live in the city and raw food is expensive and hard to find and if my apartment has an inviting kitchen, then it may take longer than 30 days and definitely more effort.

Can we state all the variables that enter in habit formation with proper units and state a general formula?