My favorite rubric for this is: give yourself permission to put the book down and never pick it up again at page (100 - your current age). Which means that, yes, once your turn 100, you're allowed to judge a book by its cover.

I don't mind taking a long time to complete a book if the story is good. I just re-read a couple of Peter F Hamilton 1200 page tomes that took me over a month to complete (Pandora's Star and Judas Unchained). However they were fun and I was immersed in the world building.

There are other far shorter novels I have abandoned after 70-80 pages because they're just not very good.