Hey, I've built a project over the course of a year. I've put in a lot of hours into it, and it has come out great. A bunch of people use it, and a bunch of people write plugins for it. However, I have since moved on to different languages, different styles, the codebase is dirty and hackish, and I'm not sure I want to continue working in the framework I built it in.
When do you know that you should shut down a project and move on?
Once you have lost inspiration and a project starts to get boring and you are to the point you can't get much or anymore experience from the project it's time to move on. If a project's codebase is too screwed up to fix then it's time to move on. If people use it and people write plugins for it, I'd personally make the project open source and turn it over to the community to continue otherwise just end it.
Find a new maintainer since it's already open source. Ask for a volunteer. If they like it they will surely jump at the opportunity to maintain it.