https://www.amazon.com/gp/help/customer/forums/kindleqna/ref=kindle_help_forum_tft_tp?ie=UTF8&cdForum=Fx1GLDPZMNR1X53&cdThread=TxUEAC1I367PZF

 

Regev – You can reset the time remaining calculations on your Paperwhite as follows:
1) Open up any book
2) Search for ;ReadingTimeReset
– include the leading semicolon, and note that it is case sensitive and there are no spaces
3) You will get 0 search results
4) Hit the back button/icon.
Your books will now show “learning reading speed”. After a bit of reading your speed, and thus time remaining, will be recalculated.