Sometimes it is obvious when a painting is done. Other paintings evolve over time.
I always err on the side of painting too little rather than too much. It is easier to add than to subtract, and sometimes the strongest statements are made with the simplest of gestures.
Over time other paintings reveal flaws. The colors may appear flat, the composition may be off, the concept muddled. Not that an abstract painting has to be coherent or rationally understood, but in some cases it is best to move on, to seek higher ground, to attempt to pull the painting out of the abyss.
Sometimes simply turning a painting on its side leads to revelation.
Here some before and after photos of canvases I have recently returned to. As before, I now consider them done.