When I read, I get invested in the characters, so naturally, I love series over standalone books. That being said, there is a certain point when series can be exhausting/intimidating to read.
The most important thing about a series for me, is making sure the story still has a strong reason to continue. I find this applies to TV series as well. There’s only so many, death/resurrections scenes that I can allow before it becomes tiresome and pointless. Throwing more things in a story to solely to extend it, doesn’t make it better. There needs to be a reason, a purpose, for things to continue other than the author wanting it to.
I prefer when stories are wrapped up at a high point—a bang! —not a drawn out, continuous lull. If you plan on creating a plot written over a series of books, figure out the overarching arc. Not to an extreme but have a basic understanding of where things are heading and the resolution. When things reach the end, ask yourself, is it pertinent to continue? If your characters have come to a nice wind down and you still wish to write in the setting you created, there’s always prequels, or sequels to be made in a future timeline. Explore a different set of characters. Another country or city in that world.
In a short answer, yes, a series can be too long. The numeric number doesn’t matter to me as much as the drive of the plot/characters. If the author can maintain my interest and believability, I’ll read to double digits and beyond.
What's the longest series you've read? Were there times when you thought it dragged on, or did the author do a good job of maintaining pace and urgency?