Bleach will change the color of a surface if it is left on it for too long. You shouldn't keep bleach on the surface for longer than eight hours unless you intend to change the surface's color. Remember that bleach contains water, which evaporates and dries up. Eventually, poisonous sodium crystals are all that are left. If you leave these dangerous crystals lying about, your pets could come into contact with them and lick them, possibly harming them.