It's too hard to generalize, because it all depends on the author. I just grabbed a couple of Terry Pratchett's Discworld
novels (mass market paperbacks) off my shelf to check their page counts. They ended up being around 350 to 400+ pages each, and I know that I wouldn't want to lose anything out of them. The high page count doesn't bother me because when I read them, it all flies by so fast that it feels like it was only a hundred pages or so. On the other hand, I just finished reading Snarleyyow
by Frederick Marryat, and the story didn't feel like it really got going until about eighty pages into it. I would have liked it better if those first eighty pages had been condensed into about fifteen.
Then there are novels like Les Miserables
, where hundreds of pages could be cut without seriously impacting the main storyline. Get rid of the insanely long, rambling digressions about the convent, and Waterloo, and the sewer system, and all the rest, and the main characters can continue on about their lives and deaths as if nothing happened. However, if you did that, it would be a very different novel. Hugo made going off on tangents into an art form. The random fifty-page-long digressions become an end unto themselves. You might not want the life story of nearly every minor character who has a single speaking line, but they're there anyway, providing depth. Trimming away the excess might improve the narrative flow, but it would harm the work as a whole.
By the way, I like your Watership Down