I think it depends on the habit. Stuff that you repeat every day and is small and relatively easy to do (for instance, washing your hands the minute you step through the front door) I can see only taking 21 days.

Other things which are either not done as often (exercise three times a week) or else which have to be broken down into component steps (write for an hour each day, which takes planning and so forth) would probably take longer to become an ingrained habit.