For me, it isn't pages so much as it is percentage. I've always figured that if I make it about one-quarter of the way into the book and it isn't grabbing me, then the thing just isn't for me.

Oooh great question! Hmmm....I'd say probably somewhere around 40-50. I'm an optimist but, if the story hasn't gotten going by that point, it's not going to anytime soon.

For fiction, as few as a dozen. I am a painfully, frustratingly slow reader (a 200 page book that might take 4ish hours can take me two to three times that, so I am acutely aware of the time invested in everything I read). Plot is not the primary reason for me to read fiction; I'm more of a character person. So I'm far more Crime and Punishment than War in Peace.  But it is the language the writer chooses that determines if I will finish it.  If it is clunky or erratic or just seems unnatural to the writer, a dozen pages is plenty.

I'm more forgiving if I'm reading for information, but if the book is a chore, then I won't go past a third.

It depends on how boring I find it. I usually go for 50 pages before I decide to deep six it.