http://www.fdic.gov/bank/analytical/overdraft/

They state that the sampling wasn't random and shouldn't be extrapolated, but for the banks with automated overdraft protection that they studied closely, 4.9% of customers had 20 or more overdrafts per year and paid 68% of all non sufficient funds fees collected by the banks. An additional 4% of customers had between 10 and 19 overdrafts and paid about 16% of the fees (so as a group, people with more than 10 overdrafts a year paid 84% of overdraft fees).

The people with 20 or more overdrafts averaged about $1,500 a year in overdraft fees.