>>96317011
Don't start with the intention to make it a long campaign. Focus on coming up with a strong concept for the campaign and executing it well, and see how long things stay fresh and fun like that. If it feels like you're running out of steam, start to think about how to wrap things up. If the GM keeps having ideas and players keep being interested and engaged for a long time, that's great, but it's better to have a short campaign that ends on a strong note than to stubbornly try to keep one that's outstayed its welcome. So basically just don't make campaign length a goal, just keep it running as long as it's fun and figure out a satisfying way to end it when you start to run out of inspiration.