Thanks for all the answers, a very interesting discussion so thanks to all of you! I would love to hear more opinions, and I also have one more question: how much do you exercise your adult cocker spaniels?
When I say "she goes crazy in the house" if not exercised more than 20 minutes or so, I mean running around / jumping up on the sofa, etc. Also, she just seems more HAPPY when she has been out for longer. I had heard about the 5-minute rule of course, but I also take advice from the veterinary, and I spoke to a woman in the Norwegian Spaniel Club (as I live here in Norway.) They both said that: this is individual, you know your dog best and just use common sense, and see how your dog reacts to it. You can try and sit down and relax for a bit, and let the dog decide the pace you walk at (sorry if my English has some faults..)
The worst thing you can do to your dog, is too little acitivity and too much "boring" time in the apartment.
PS. I dont take my dog on 1 hour walks EVERY day, but perhaps 5 times a week, the other days she gets a bit shorter walks.
When I walk my dog in the forest, I dont walk particularly fast. Still, she runs back and forwards, probably walking 2-3 times as much as I do on the same walk... This, she decides for herself. Also, when I sit down for a while, she continues to run around and looks like she doesnt need a break. I keep her off-lead most of the time, as we walk in the forest.
I also agree with those of you that say that dogs are animals, originating from the wolves. They are "built" for more active lives than we humans can offer them.
I sometimes take my dog to a friend, who lives on a farm. After playing for an hour with her dog, my pup is much more tired than after we have walked for 1 hour.