IF I like the series and it keeps "true", the longer the better!!!
A lot of people probably are proud that they do more reading than TV watching, but honestly... I read FLUFFY books. I'm not growing my brain with Stephanie Plum, Sookie, or even Tempe Brennan and Kay Scarpetta. I'm only entertaining myself!
The reason I really like a long series is I can go from one book to the next seamlessly. I already know the characters and care about them. It can be 11:30pm, just finished one book, and move on to the other yet fall asleep before midnight. That's what I like. I don't care if it's unimpressive, but when I read for fun, that's all it is
For fun, and I like moving from one book to the next effortlessly.
What I do not like is when a series is really good, but then "jumps the shark". That happened to me with the Besty the Vampire Queen series reached like #10 or so. And, yes, I do realize to say that the vampire queen jumped the shark seems like an oxymoron as if she hadn't already... but trust me
The series was SO good and with storylines and characters you could easily relate to. Then it got into this time travel situation -- to include very long passages from the prior books -- and after a few nights of feeling like the book was a school assignment, I finally check out. I don't regret having spent time with the first many books, but I wish the series had either wrapped up, or stayed away from that shark tank!
It's also fun to read the same author back to back, even when not a series. I read (listened to) most of Jonathan Tropper's books back to back and that was fun because I was into his "voice" even though each book was about new characters.