How much time do professors have to do research on their own? As part of their job, professors have to take care of getting funding, prepare classes, advise students, fulfill administrative tasks and attend to various meetings/conferences.

How much time do professors have to carry out research on their own (i.e. excluding the above-mentioned tasks)? 

I am especially interested in the field of computer science > machine learning, in the US, in an averaged-size university.

I am mostly looking for studies that try to quantify how much time professors have to carry out research on their own.
I found this small-scale, not randomly-sampled survey from Boise State University:

Warning:


  All charts below are from TAWKS Phase 1 Stats, initial survey of 30
  higher ed faculty from Boise State University. While findings are
  highly suggestive, they do not represent a random sample.


Answer to question:


  Only 17 percent of the workweek was focused on research and 27 percent
  of weekend time.


Graphs: