Does it make a difference how long you exercise for?

Well, yes and no. Basically, it's fine to do 10 to 15 minutes at a time, 3 or more times in a day, if you can do that. You really don't want to do less than 10 minutes for most cardio though, as it's hard to properly get your heart rate up to where you want it to be in less than 10 minutes.

There have been numerous studies that have shown empirically that you can exercise all at once, or break it up throughout the day with very similar results. The key is to get at least 30 minutes of moderate cardiovascular activity in per day for 5 days a week, whether that's 3 10 minute sessions, or 1 30 minute session doesn't really matter.

When you are trying to lose weight you should try to do over 20mins - the first 20mins is burning up the free floating glucose (blood sugar) after that you are burning fat. Good luck

takes far longer than 20 minutes. And that's not really the whole story, you never burn up all the glucose in the blood, nor would you want to, blood sugar does not equal fat, and it is what powers our muscles (along with every thing else), in fact, when we use body fat it's first converted to blood sugar for transport.

EDIT:

I think I know the thought behind this. There's a lot of rumor and innuendo out there with regard to when we start to actually "burn fat". Truth is, the body isn't that simple. You're body is always trying to burn fat stores, and always trying to store more fat as well, these biological processes almost never really "shut down", they do however slow down in certain conditions, the liver gives you glucose (blood sugar) to use as energy, that's transported by the blood stream. The longer we work out, the more DEMAND for energy there is, at a certain point the liver no longer has adequate supplies of energy required for work, so other mechanisms are enabled, like burning extra fat, this is done IN CONCERT with carbohydrate conversion, not in stead of. There are cases where this doesn't happen, like when a person becomes ketogenic (very low, or no carbs), where they convert amino acids and fatty acids (protein and fat) into energy, but this comes with it's own set of rules and should be only undertaken once you are well aware of them and willing to give up quite a bit in the way of food types.