Habit Formation

Matt's comment reminded me of a recent controversy I encountered with a colleague. Here it is: What research is there that indicates how long it takes to form a habit?


I spent a bit of time on an internet search of said topic, and came up with claims of:
10 days
21 days
28 days
30 days
3 months
10-21 days
..with 21 and 28 days/repetitions coming up more often than the others. A lot of the occurrences were on self-help and weight loss websites. There were plenty of lead-ins along the lines of "experts tell us..." and "research has shown...", but no concete references to which researchers or what specific findings. Maxwell Maltz and Abraham Maslow were mentioned, but I can't find anything in their work to pin a timespan on habit formation. Is this habit-timeframe stuff just folklore? Does habit formation vary too much to assign a set number of repetitions for adoption?

I do know that it only took 2 doughnuts for me to form a Krispy Kreme habit.