I find it to be pretty accurate. It does seem to change, though, depending on the book I'm reading. Which only makes sense: it takes me longer to read a section of War and Peace than the same length section of a light cozy mystery. Initial estimates are generally off at first when I switch between the two. I do like to use it before bed -- to decide whether or not to go ahead and start the next chapter, for example.
It also changes
as I'm reading. Frequently, I'll start reading a section and it'll have a number and then after I've turned a page or two I see that the number hasn't changed at all, or that it's gone way down or way up. So it adjusts as you go.
Also, it really does depend on how fast you 'turn pages*'. So if you're in the habit of reading and putting the kindle down without closing the book, and just letting it fall asleep on its own, it'll think it took you 10 minutes on one page. The next time you start reading, that'll be figured into the calculation.
In your case, maybe you just finished a book that you were able to read pretty fast, or were almost skimming. So you're turning pages at a pretty quick rate. The kindle thinks that's how fast you always read. Then you open a new book that you have to pay more attention to. You're turning pages much slower. So it said '10 minutes' to start with, but it's really going to take longer. It should adjust, though, as you read; it does for me.
ALSO . . . . it counts 'to next chapter' according to how the book is formatted. For books with chapters and sub chapters I find it less useful as it usually counts to the next full chapter. So it might say I have 2 hours to the next chapter but I know there are subs and they generally are about 10 minutes of reading for me. I suppose it could happen in reverse as well: it says it's 10 minutes to the next chapter, but that's just a section of the chapter and the real end of the whole thing is actually 30 minutes of reading.
*using 'page' in the sense of one screen on the kindle.
