I don't know whether the length of adverts on US TV are longer or not. I do know however that they are far, far more irritating.

I would far rather have a three/four 3 minute breaks an hour (with one or two of them coming between individual shows) than the American approach. How bloody annoying is it to sit down to watch a show to get two minutes into to it to suddenly find yourself in a (short) ad break again. At least in the UK when you sit down to watch something you know that you will have 20 minutes to watch uninterrupted and then a handy break long enough to make a cuppa in