I read and write fantasy. Most of the early fantasy I read had been written in three book series. Most epic fantasy followed the standard set by Tolkien. After all if the master could create a story the scope of The Lord of the Rings, would would anyone need to drag the story out for more than three books. But some could argue Tolkien's saga was actually four books if you include The Hobbit. Then along came Robert Jordan and his Wheel of Time series and a new idea of how long epic really is and it's not done yet, outliving even its creator. Terry Goodkind's Sword of Truth series came along to challenge Jordan's record and he's still going with it.
My first fantasy series is four books long. I planned it for three but the complications took longer to bring to a head than I expected when I started it. I'm currently a book and a half into my second fantasy series and expect it to also be four books long.
How long do you like for your book series to be as a reader or a writer? Have you fallen out of love with a series after you'd invested time reading the early books in it?