Quoted^(ref) Platinum Trickster Negi-kun (on 9 September, 2009, 10:32 PM PDT) —

Yep, it really depends on both the quality of the series and the storyline, the characters, all those tiny details. Me, I almost stopped following Negima! by the time it hit book 20, but the series is already about 3 times longer than Love Hina was (and LH was 14 books), and I'm still following it as every new story arc just keeps me hooked. Not to mention it's got awesome art as well. :> 

Short series also have their own charm, but for long series, I think it's more based on what makes the fans come back for more.