Goo has an interesting survey up, ranking popular Shonen Jump manga that readers give up on partway through. With the chapter-load of a lot of these series, it's tough to blame them. Have a look at the list below!
1) Naruto (561 chapters, ongoing)

2) Kochikame (Kochira Katsushika-ku Kameari Koen-mae Hashutsujo) (1,678 chapters, ongoing)

3) One Piece (644 chapters, ongoing)

4) Prince of Tennis (379 chapters, original run complete; 64 chapters in new series, ongoing)

5) Bleach (470 chapters, ongoing)

6) Captain Tsubasa (roughly 488 chapters across multiple series, ongoing)

7) Katekyo Hitman Reborn! (tie) (358 chapters, ongoing)

7) Hunter x Hunter (tie) (322 chapters, ongoing)

9) Gintama (375 chapters, ongoing)

10) Jojo's Bizarre Adventure (over 104 volumes, multiple series; JoJolion ongoing)

What about you? Have you given up on these or other popular, long-running manga despite the quality?
Via JamGuru