It actually depends on my job respectively my projects.
When traveling within Germany, I usually take the train.
On the train, I'm only doing 3 things: Eating, sleeping and reading.
More or less the same in hotels: Eating, sleeping and reading.
So, when at home, I'll read 2 to 3 hours per days.
When traveling, it can be 5 hours and sometimes even more.
That's why I love my tablets: With a tablet as your travel companion, you cover it all: Reading, watching moves, surfing the web, writing emails.
If I include reading and writing emails in the equations, this easily adds another 3 or more hours per day.