I reading on my K4 heavily. I get 15 to 17 days before I need to recharge.
I read about 3 to 5 hours sometimes 8 hours if I can't put the book down. I usually read during the week as weekends hubby is home.
My K3 just sits on its shelf lonely. I have found if you don't use a K3 you get two months before it needs to be charged again.