Roleplays should last as long as the participants wish it to. 

Starting a story with an idea in mind of where to start and where to finish is good, but leave room for continuations (timeskips, spinoffs) if you're really feeling the chemistry with your partners, and wish to extend your partnership beyond the main story. I remember starting a story as a one-shot idea with a partner, but we loved our chemistry and the potential of the universe so much that we've decided to expand the story and created spinoffs within that same universe.