Very long. For the first 300 pages I was really enjoying it. My kind of novel: some science fiction (AI surveilled society), some time travel (to ancient times), some style (I had just finished Chandler, and as first Harkaway channeled some of that), lots of wordplay, and very intellectual. But… it went on and on, and after awhile I was jumping chunks of 3-4 pages, nope, nope, nope…. So where Jorge Luis Borges could have crafted the same story into 40 pages, Gnomon is about 670. I don’t regret spending a couple weeks on it. But you might.