If it's been superseded, then that's one thing... but if not, then why not just work on it "whenever"? Having side-projects around is great for if you get burned out while working on a main project. Some of my projects are ages old but I push them forward every once in awhile... they may get finished someday and they may not, but closing the door only ensures that they will never get finished no matter what. My oldest project was originally started in 1987 and is still being worked on to this day.