Saw this today, thought it was pretty sweet and applied to us (even though we won more than 20 games instead of less than 20). Supports Kahn's idea of this process taking 2-3 years at least.
As you can see, most teams do not stick around the deep cellar for long: the average win total for teams following a 19-wins-or-less season is 26.9 wins. If you consider teams with records that fall between 38-44 and 44-38 to be average, then it takes (on average) sub 20-win teams only four years to get to that level. One team -- the Mavericks -- took some eight seasons to get from awful (11 wins in 1993) to better than .500 (53 wins in 2001). Every other sub 20-win team got there more quickly.