Why stick with it? My view is that the main purpose of side projects (for me anyhow) is to learn and improve my skills and to open up to new ways of approaching things. But even if your goal is to work it into a side business, I still would argue that sometimes sticking to the same side project too long can be a bad choice. I have a fried who worked on the same side project for 7 years and I think it stifled his creativity because he was always focused on the same narrow set of problems associated with his project.

I am simply suggesting it is possible to go too far in sticking to the same side project. For me I usually consider a side project has run it's course if I have learned as much as I needed to on it and then I try to move on to something completely different.