Well, I think there's been a lot of study done in this area - whether or not you are working in your 'fat burning zone' and so on. From what I've read, the longer you work out, the more calories you burn, even at a lesser intensity (Of course, you don't want to be just meandering!)... and since weight loss is, basically, calories in versus calories out - well, you see my point.
I do know that new guidelines have been issued suggesting that 30 minutes of cardio three times a week is not enough to lose weight - the new suggestion is 60-90 minutes three to six times a week!
Gives you an idea, though.