Robert Townsend, Director of the Washington Office of the American Academy of Arts & Sciences, kindly sent along this new data. He writes:
Instead of the usual calculations, I purchased numbers for the time students spend in the program that confers their degree, which more accurately reflects the focus of many recent reform efforts. The key findings are 1) that even by this more precise measure, the median time humanities PhDs spend in their programs is a year longer than any other field (6.9 years in 2012, as compared to 5.9 among all new PhDs), and 2) the difference between the fields seems to occur at the coursework/exam stage—not the dissertation stage, as many reformers have assumed. Sadly, we could not purchase numbers specifically for philosophy, but I hope you will find the broad trend for the humanities of interest.