If the sales are good, Apple will say after the first weekend or week. I know in the earnings they aren't going to break it out, but if it makes a splash, you will hear a number from Apple either the first or second Monday after launch. If you hear no numbers at all, Apple risks the watch being labeled a bust. They are now in a position where if a product doesn't break records at launch, it is a bust in the eyes of many (not myself).
I think it will be a little above 1 million in the first week as long as that many hit stores. I do think Apple will ensure there are supply constraints, for the reasons I stated above. They can't afford for people to be able to easily get one Sunday after the launch, and not report gigantic numbers.