I have many hobby projects, ideas, partly-built things. I have some that are very near to completion, and others that are probably a lifetime's work.
I tend to work on them in short stints: a few nights, sometimes like a month or two. And then set them aside as I burn out on that idea, or come up with a compelling extension/revision/use-case/etc for a different one.
It's really crucial to document where you're at in this model: you need to be able to pick things up again without too much time to figure out where you were. I have two things that help with that:
Firstly, a README.md file for each project which includes my roadmap, with stuff crossed off as I get it done. So I can pull it up and see exactly what's next and what I just did at a glance.
And secondly, I keep a (large) journal.md file, divided by date, where I write down what I did. Especially stuff that I needed to research to figure out. That includes literally copying command lines out of my shell and into the journal. It's just a massive bulleted list, but when I restart something, I can quickly search back to the date of the my last commit, and see exactly what (and how) I did the things I did in that burst.
Combining my roadmap/README, journal notes, and Git history generally means I'm productive again in about 15 minutes.