I think the time it takes to make or break a habit is less important than how fast your subconscious can make good or bad associations as it experiences the habit.

Take quitting smoking for instance. I tried quitting cold turkey a few times with great discomfort and would eventualy start up again. I later found out that I was instructing my subconcious that quitting smoking was bad (because of the discomfort) and smoking was good (felt good when I smoked).

I found a quit smoking program (from France) that reversed my old assumptions by teaching my subconscious to feel bad about smoking (through aversion techniques) and feel good about being smoke free (through guided imagery).

The program was simple but the effect was most potent. I still to this day have an aversion to cigarettes and hate the smell of them.

This technique now forms the basis of all the new habits I make for myself.