When do you consider a track 'finished' ?

I set very high standards for my own music, so when I listen to all the tracks that I've made so far, I don't really consider them 'finished' -- there's always something that could be mixed better, another element that could be added, etc. But at the end of the day, I know that none of the tracks I have so far are release-quality and I wouldn't send them along to labels and such. So should I just call it a day on these once I'm done arranging them and move on to the next project? Or is it better keep tinkering until I'm happy with them? Is it worth expending the effort to do a rough master and such on them, or do you all find it a better use of your time to just archive these projects and start new ones and keep going?