My best side projects have been ones that I can walk away from at weeks at a time and pick back up when I'm interested with no ill effect.
Projects that require constant attention always burn me out after the initial building enthusiasm ends. It basically becomes a 'job' at that point, but with no real upside.
Likewise. I venture to say that a Side Project can only be defined as a Side Project if you are able to walk away from it for weeks at a time.