There’s already an existing feature suggestion discussing that option. Please comment/vote in that thread in order to avoid distracting or cannibalizing votes.
Currently Plex consider a media file to be played if the file is played to about the 90% point. This is OK for movies, TV and most other videos BUT it is totally inappropriate for longer media like Audiobooks.
There are many audiobooks that have a length of over 20 hours and not a few that are above 60. So, if you are playing a 20 hour audiobook and you get to the 18 hour mark and you need to stop for any reason when you go to resume it the start place will be back at the start of the book and …
I’ll therefore close this thread as a duplicate.