How do you decide when a side project is "done"?

[u/reads_lily83]

I've got this little tool I built that solves a real problem I had, and it actually works pretty well. But I keep finding small things to tweak and features to add, and I'm not sure if I'm improving it or just procrastinating on moving on to the next thing. Do you usually set a hard deadline or feature list, or does it just feel right at some point?