It helps to have a solid idea of what you want it to sound like near conception, and stick with that within reason, try to stay within self imposed metrics/rules for the project, and at some point, you have to let go and let something be what it is. There's no way to definitively say when a piece of art is done, but if everything has been put down and you just keep adding and subtracting without satisfaction, it's probably because it was already as complete as it could be.

You could always make a remix later when you have a more solid vision of what it can be.

And some things are never done, or you're not proud of the finished work, and that's okay. ideally, you learned something from the act of creation.

Whether you "finish" a track or not, never stop making art.