I don't think length is such an issue, it's a matter of the quality of content.
Personally I didn't find a single one of the FO3 DLCs amazing. They were ok, but they gave you the sense of "meh" where it was something extra to do, but not something you were dying to do. Like I felt I had purchased them solely for the sake of having the complete game, not because I was actually DYING to have them.
The average FO3 DLC was probably...5 hours of length, the first playthrough? Playthroughs afterwards you complete the DLC quicker of course, but the first playthrough I'd say was 3-5 hours.
New Vegas, I think the average DLC length was 7-8 hours on the first playthrough. Longer, but really not that big of a difference. Honest Hearts for example was slightly longer than the FO3 counterparts, but I really don't consider it anything special. Lonesome Road was equal to the FO3 counterparts, but I found it totally worth my money. Dead Money was slightly longer, but it's easily the best DLC I've ever played; not due to length, but because of the story and the challenge.
So in the end, length doesn't matter so much; quality does.
I'm just amazed at all the people who haven't learned that a Todd Howard "10-20 hours" probably means "10 hours" and not to get their hopes up.



muahahahaha