For me, the limit should only be that if there's something obviously planned, then it should be free, otherwise we should have to pay. After all, everyone will complain, but us gamers are a greedy lot, and though the industry does have plenty of bad as well as good, the fact remains that we get a hell of a lot for the £40 we spend on a game, regardless of how much we actually buy it for and whether it's late or on release or pre-owned or whatever. From that,. because I'm already getting more than my money's worth when I buy a good game, even at full price, I'm more than happy to shell out more money for DLC, if it's any good. And before you accuse me of being someone who can obviously afford it, I can't really. I work a part-time job in a supermarket and can't afford rent where I want to live, so I'm living with my parents until I get a full time job. I've only just left university too.
But I digress. My point here is that there has to be a limit, but most DLC should be paid for, and fairly too. For example, the Assassin's Creed games. Now, I bloody love the series, it's one of my favourite (and most played) series in my entire game collection. But in AC2, there are two DNA sequences that were planned from the start that were released as DLC. And it's even stated in the game about them, the game itself lampshades that they're missing. And Brotherhood does the same with the Da Vinci Disappearance. That, though I did buy for the sake of the story and game completion, is basically Ubisoft screwing us over as customers (though as I wrote in an article a while back, and as most gamers should know already, it's hardly the first time...).
Then you come to something like the Defiant and Noble map packs for Halo Reach. Those were extra multiplayer maps, and I feel no problems paying for those. The reason? THEY WEREN'T PLANNED IN DEVELOPMENT. Most DLC, as it happens, isn't, either. The thing is, after all the main elements of gameplay and design are complete, the game needs to go through testing, bug fixing, and then through the publishers, before being shipped. That's several months of downtime for the rest of the development team, who can either go and start working on a new project, or they can start working on extra content. That content is what becomes DLC, and if it doesn't take long to make, it's what becomes known as 'Day 1 DLC'. So, people think that we should get that extra stuff for free because it was released on the same day as the gold product? That is actually completely unfair, towards the developers rather than thecustomers. This is extra content that people worked hard to make, for the benefit of the customers, the gamers, rather than any desire to screw money from people. So yes, we should pay for that extra content. Not to do so is screwing the hard working developers from revenue that they fully deserve, and not paying is simply being ungrateful, selfish, and self-entitled. Though as I've stated before, that's already a major failing of gamers anyway.
TL; DR: Gamers are one of the most selfish, self-serving, self-entitled demographics on the planet, when it comes to their chosen form of entertainment.