In a manner of speaking, do not stop painting until:

--your characters come alive and begin breathing and moving on your canvas
--you see day turn to night and back to day again on your canvas, showing up all the different highlights and shadows
--you hear voices and sense the presence of other characters behind the walls that you painted on your canvas
--other characters step into your canvas, walk across it, and exit
--your studio is suffused with an unusual, bright, light
--you see a grid pattern superimposed on your canvas at night, when all the lights are out.