David Roth has no rules for when a painting is finished. As he mainly uses oil paint which needs time to dry, he usually works on several different canvases at the same time over a period of several months. Then he puts them away, to let them breathe and dry. After he left them alone for a while he looks at them again with a fresh eye, to decide what to do.
Sometimes a work indeed turns out to be finished, more often it needs more work or a new start. For Roth it is like being in love: you work on a piece and you feel like you absolutely love it. But at some point you need to take a step back, let it rest for a while, to see if it was not just a crush, but whether you are actually in love.