For years now I've held this theory that any good series of novels should never go beyond five books. As with any rule, there are always exceptions, but from my personal experience, the five book rule is a good guideline.
Think of it this way. Have you ever fallen in love with a television series? A classic might be Buffy the Vampire Slayer. This popular show ran for seven seasons. Like many, I watched every week. For me, though, the show started it's slow decline after season five. Do you get the connection? Instead of ending and leaving watchers wanting more, the show fades away. Now think of shows like Farscape or the original Star Trek. Those shows had runs of four seasons, left people wanting more and have a significant cult following.
See what I mean? Think of each novel in your series as a season of your favorite television serial. Are you going to end on a high leaving your readers wishing for more? Or are you going to milk your success for all it's worth until the plot and characters are so worn out that your readers don't even bother any more? Remember, in an era of instant access, many people just don't have patience and become bored very quickly.
In junior high school, I found the Xanth series of books by Piers Anthony. I thought they were the greatest thing ever. I bought the next one and the next one and the next until book 12; I got bored and never bought another. According to Wikipedia, he's working on the 36th. Frankly, I'm amazed there are still enough readers to warrant continuing!
My mom likes to read murder mysteries. Those tend to go on well past what is, in my opinion, their freshness date also. I understand we get attached to characters, as both readers and writers, but there is something to be said about the thrill of finding a new love; saying a sweet goodbye to the old.
As I mentioned above, there will always be exceptions. Series that are still fresh and exciting well past book (or season) five. But I maintain that anything much past the five mark is running a risk of losing that spark and losing readers/viewers.
What do you think?