How long to you persevere with a book that you haven't got into yet

Three chapters

Depends on the author. Sometimes a lot of background has to be laid out before it really grabs me. I speed read that part.

I usually try and finish them, especially as some of my favourite books I still wasn't sure about when 1/3 of the way in.

I give it three chapters.
If I've nothing or no one to be interested in I bin it and do something better with my time

Forever, i put it on hold till later probably after i die!