I hate to buy DLC. I do buy it for games that I play a lot (Skate 2 comes into my mind). I haven't gotten it for Borderlands mainly because my character got "fucked" when I played online and people just rushed for like 100 missions so I lost a huge chunk of the game and put it to rest. I do want to try out the DLC for Borderlands, but I won't buy it before I finish the main story. So in a timeline, I'd say that DLC should be released after the average time being spent on the main quest\story. Release it too early and people might not buy it, release it too late and people have forgotten how fun your game is.
I don't like MS points because they sell them by the 1000s, while games are variations of 400, 800, 1200 etc. I always end up with some retarded 385 points, and I want a DLC that is 400 points, oh well, I guess I have to buy another 1000, 2000 or 5000 points just to get those dumb 15 points which is what I really need. I don't understand why full games (Games on demand) is a proper currency, while the DLC\etc. uses MS points, it's a hassle.