There are weeks when I mostly read dead tree books. There are weeks when I only use my PRS-505. Thus the ranges.
From an hour and a half a day on three days during the week (while pedalling away at the gym) to 10 hours a day (when I get lost in a book that I HAVE to finish right away).
Average over the last three years is probably around 2 hours a day. I'm still using the the original device that I bought used on Ebay.