"...an epic fantasy novel..."
Right about there is where I stop.
Kidding, kidding... No really, I almost never don't finish a book. The last ones I can remember abandoning before completion were:
The Moon is a Harsh Mistress - Robert A. Heinlein, I started and stopped this one in 1999 about half way through. I love Heinlein, I've read just about everything he's ever written, including his insanely jingoistic travelogues, and cuckoobananas Patrick Henry "WE MUST BUILD MORE NUKES TO EQUALIZE THE EVIL COMMUNIST MENACE" articles. I even finished "I Will Fear No Evil", possibly the most idiotic book ever written, but I finished it. Couldn't make it through the relentless speechifying of Mike the computer in Moon though. I even finished Glory Road, his one stab at epic fantasy that I disliked immensely.
Better Than Life: Grant/Naylor, 1994 This was before Red Dwarf was even on TV regularly here in the states and I'd watched the first three seasons (I think it was the first three) in college in the UK. The book, based on the first season episode, was unreadably awful. I made it all of 110 pages before it went into the recycle bin.
Harry Potter and the Prison of Azkaban, J.K. Rowling. Made it through the first and was unimpressed, was bored and annoyed by the second and couldn't believe how much people lauded the Scooby Doo quality plot, absolutely hated the third. I gave up on this one before her 130 page catch up and retell the first two books segment ended. Hated every page of it.
I'm pretty sure I've completed every other book I've set out to read, and I've read some stinkers (See above "I will fear no evil") but I rarely just drop the book. Mostly though now I read either classics or nonfiction. I don't like contemporary fiction of virtually any sort enough to seek it out for more than the occasional foray. I mean, I read a little David Foster Wallace, Oblivion was a collection of shorts which had a couple of good stories in it, I read Palo Alto Stories by James Franco (that guy who played son of The Green Goblin in Spider man...) and it was okay albeit a little repetitive storywise. But mostly I read nonfiction now. I hit a few podcast novels, but my opinion of them isn't worth dredging up really, and I usually listen to those until the end even if I vow to never listen to another anything by that author again.