I believe that they go 6 years between changing the edition or textbook they are using. I don't believe the actual text books last that long. Surely some last longer and some don't last nearly as long.

The 3 years I chose was based on the average time that textbooks lasted in my high school. (I worked in the text book department during school, and that was the time period, though it may have been 3.5 years.)

How often would schools like to get new books?

At high school they finally found the money to replace the history textbooks once the books were the same age as the students studying them (the actual physical textbooks were at least 8 years old when I was a freshmen in HS).