When the character no longer works the way he is, whatever that may be. I mean there are exceptions, like if Clark Kent or Bruce Wayne were slave owners, that wouldn't exactly be cool and might be just a LITTLE taboo, so that'd need to go, but on the whole the things that work ought to be left alone...until they don't work anymore....whenever that may be.
Some I am, some I'm not. Some are good, some aren't. Some worked, some didn't.
Jesus Chris. I'm not talking about updating the character like giving him straight leg pants instead of bell bottoms or letting him have a cell phone instead of sending telegrams, that kind of thing is obvious and goes without saying, I'm talking about changing what Superman is, as a character, as a work of art, as a visual, as everything that makes him iconic and identifiable - those things must always remain unless the character is on life support and in danger of being wiped out, then you can come in with your little racial changes and your costume and power and name redesigns and do whatever you want to get it successful and popular again.