The time between charges is probably the best metric for these devices, not page count. As others have pointed out, the page turns are just one factor, the and the page count numbers unfairly remove those factors (so it is an optimistic estimate for a speed reader who can churn through thousands of pages per day).
5 days is a bit low... if no music is being used, I'd say the battery is probably starting to go south. A fresh battery and you are up to snuff. My 505 is still holding a 2-3 week charge after 2 years, so I consider myself lucky.