When or if to give up on a book is a personal decision. Personally I try to allow an author 20% of the page count if the book is well written, less if it isn't. I wish I'd applied that rule to Seveneves, but I didn't because it had been so heavily recommended. By the time I'd finished it I actually felt angry for having my time wasted.

My opinion of Stephenson is that he is smart guy and an able writer (particularly of non fiction), but also that he is an undisciplined writer.