In 1994, the Washington Post ran an editorial about the alarming arrest rate of DC police officers. It compared the percentage of DC officers arrested to the arrest rates for other departments, including NYC, Detroit and, IIRC, St. Louis. The average percentage was around 2%.
I've tried in vain to find national statistics, but it seems that internal affairs departments don't often make arrest data public.
Anecdotally, Milwaukee has a department with roughly 2,000 officers. There's been about a dozen arrested this year for a variety of crimes, so at year's end the percentage will be in that 1-2% area.