Do you think a series can ever be too long?

[Tobi Adefila]

Of course! a series can be too long. I feel the longer a series the more interesting it gets. Though that's doesn't apply to all series, there are some series that one will come across and be like "Why do they have to go that far, what's the point" such series is just a waste of time.

[human reader]

An anthology series where each book is its own story can go on forever, like Goosebumps (although Goosebumps has some books with direct sequels).
A series that isn't too connected, like maybe a lead character but each time a new setting or at least new plot, could go on forever.
A heavily connected series like Harry Potter shouldn't go on infinitely IMO. Maybe the rare writer can keep a series worthwhile 20+ books with the same focus but I think most would prefer they wrap up 7-10 books max.

[Tomy Chandrafrost]

Yes. If it's too long, people will have difficulty following and get bored. I'll take an example from the soap opera in Indonesia titled "Tukang Bubur Naik Haji". I think this soap opera has had thousands of episodes and ran for 3 years until people got lazy to follow it.