How many pages do you read of a book until you decide its worth reading or not?

Depends how long of a book it is. If you can't set up interesting characters or a premise within the first two or three chapters (unless they're oddly short chapters) I'm probably going to stop wasting my time on it. I don't have the time, hell I mostly listen to books now because there's more time for that than reading one. That said there have been times I've gotten over the hump of a slow introduction and it pays off.

Non fiction is a different beast. It really comes down to how well it's written and the information is presented.
 
Depends on the book. One time I noped out after working my way through a paragraph-long first sentence. Another time it took me halfway into the book before I finally realized the plot was only happening because every character was an idiot, reprehensible, or both. Since there seemed to be little chance they would all die in a fire, I was done.
 
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