One of the great features of the newer Kindles is that they tell you how much time is left in your chapter and in the book. It calculates this based on how fast you are reading. It’s one of the great features you can add when you’re reading digitally instead of with dead trees.
Unfortunately, for some reason the calculated time remaining isn’t always accurate. If you find that it’s not quite right, you can tell the Kindle to re-sample your reading speed and make a new calculation. To do this, click Search while you’re in the book and then search for “;ReadingTimeReset” (without the quotes.) Voila!