For the years 1994 through 2000, I've listed below the both the median total time-to-degree and the median registered time-to-degree* for doctoral students in chemistry. Sadly, there does not appear to be a breakdown for subfields in chemistry:
(year): (total time-to-degree)/(registered time-to-degree)
I'll be looking into the eighties and the oughts soon. I note that, for 2011, median time-to-degree was 6.9 years, with a registered time-to-degree of 6.0 years.
*According to NSF's definitions: TTD: total elapsed time from completion of the baccalaureate to the doctorate (total time to degree), RTD: time in graduate school less reported periods of nonenrollment (registered time to degree)