Currently, Microsoft's development guidelines tell developers and publishers that the optimum time to release DLC is "within the first 30 days" of a game's release. The problem with that though is that it's not enough time to gather enough data about the audience's behavior and then generate content that reflects it. Content delivered in the first month has to be pretty much finished and sitting in the first party approval queue before the actual game comes out. So right now, that first bunch of DLC we see for something is usually based on a hunch, rather than the way we actually play. For some games that appeal to specific tastes, that's easier (I guess) to anticipate. But as games are increasingly under pressure to achieve monstrously huge sales, the whole system will have to change.
via gamepro.com