That length doesn't seem that ridiculous to me. The audiobook is 8 and a half hours, and I feel like I can read faster than a narrator. Keep in mind, this is 6-7 hours of continuous reading, so how many days it takes you to finish a book will depend on how often you get a chance to read.
According to the site:
>The reading times are estimated based on a few assumptions we have taken - average reading speed (we assumed 200 words a minute) and average words per page (we assumed 300 words per page). Therefore treat it this way - as an estimate, for every individual the reading speed will vary, same with words per page - books are very different. But the bootom line is, at least you get an indication on how long you would enjoy (or not) reading the book. Have fun ;)
Frequent reading definitely increases your reading speed.
Also 6-7 hours definitely seems like too long for the first HP book; I took 5 hours to read the fifth HP book when it was released, which is approximately 3.3x longer than the first HP book. I'd estimate that if I reread HP and the Philosopher's Stone it would take me less than two hours.