Hmm...This is tough since they can live forever. Well, you can plan out what you want your story to be and then end it where you want to end it. With the immortal characters, if you ever feel like picking it back up and writing more, you can.its up to you when you want to end it.
I have found that when i get in the writing mode, there is stuff i originally planned on using that i change. also over time my stories change. its how the creative mind works, its always changing and evolving.
Always it is difficult to finish something, but if you realize a series of books, it(he,she) might develop it more, always it is necessary to bear in mind the detail of the final edition.