50 pages for any novel should be enough. In that space the author should be able to hook the reader, through their use of language, characters, plot, setting, or some combination therein.
For most books I read these days, I know I’m all in after the first few pages. Redshirts, for example had me during the prologue. The Mirrored Heavens (review) took some additional time due to pacing and wanting to get a feel for the different point-of-view characters.
Sometimes an author will face an uphill battle. A 1st Person narrator, for example. You better come correct in your first chapter with your 1st Person or I’m likely out. Overexposed conventions in sci-fi and fantasy (such as vampires and orphans-of-destiny) get even less leash.
My patience can be expanded if I know this is a writer’s first book, or the novel comes highly recommended by someone whose opinion I value. And actually, I have some friends who will only give a book 10 pages or one chapter…
This doesn’t happen as much anymore because I usually do enough research on my next read to prevent it. But the last few days, looking at a new novel, I had to invoke the rule. No, I’m not going to name the book and author here – the Intertubes has enough negativity. On to the next story.