This is tough to answer; it might be 2 hours. Might be more if I go to the gym and read while I'm on exercise equipment, rather than walking or biking outside (much harder to read then!). Sometimes I read more than one book in a day, and rarely read in large continuous blocks of time, so the 2 hours is only a guess. It's probably more like 1 to 3 hours, and 2 would be my average.
Dr. Who: "Big day tomorrow!"
"Always a big day tomorrow. I have a time machine; I skip the little ones!"