I've planned to spend 5-ish years on Blue Star Rebellion. It may potentially take even longer, though if it does, I'll be making it side-by-side with my main project.
Said main project is extremely long-running, and I completely intend to spend the rest of my working life on it. It's split into smaller, more contained 'arcs', which are self-contained stories in and of themselves... But there are so many, and the universe is so broad, that I'll never run out of material to play with. (Blue Star Rebellion is technically an 'arc' within this broader universe.)
So, no. As long as you have the drive to spend five years working on the same project, I think it's a sensible amount of time!