I usually get bored and don't read an entire series (i.e. Game of Thrones Series) The long running series I stick with tend to be in the mystery genre:
Lee Child - Jack Reacher series
C.J. Box - Joe Pickett series
Jo Nesbo - Harry Hole Series
Arnaldur Indridason - Inspector Erlendur series
Michael Connelly - Harry Bosch series
and the aforementioned Poirot series by the great Agatha Christie
On a side note, I find myself shying away from other mystery series that I've wanted to try until I've completed the ones above.I guess I'm afraid of losing track of or mixing up the characters and what happened in past books. I've only been reading mysteries for a few years and read a variety of genres, so haven't finished any of these series yet (plus, with the exception of Christie, they are ongoing). Anyone else experience that, where you feel like you're maxed out on long running series and are hesitant to start any more?