Essentially, they grow back, each time, longer and longer. The reason I know this is because I have seen it happen to a friends cat. Her boyfriend thought it funny to conduct this cruel experiment until the cats whiskers were near twelve inches long on either side. This meant that the cat considered any gap under two feet wide to be unpassable. The boyfriend naturally found this hilarious. I didn't hang around that particular household for much longer, so I have no idea how the story ends. All I can do is make some educated guesses. Two outcomes are possible I suppose: Either the cruel trimming continued resulting in the cat unable to cross even the widest threshold into a different room, or the owners are now forced to trim it's pets whiskers on a daily basis. I really should get in touch to find out.