Been mulling this over overnight and I think that, if you can honestly say your pup is in complete control of it's own walk - how long it's out for, it can stop and sniff whatever it wants, whenever it wants, and there is no playing with older dogs (only with pups the same age), no lead walking, no ball throwing - then, yes, a pup can regulate its own exercise.
I think, with a medium sized breed like a Cocker, once your approaching 1 yr you can be a little more flexible in the application of the 5 min per month rule (Honey had several hour long walks when she was 10 months - but that's only 10 mins extra on a few occassions, not double the recommended amount every day.)
Yes, the 5 min rule applies more to on-lead exercise which is harsher on developing joints than pottering around a field, but it is worth having in the back of your mind.... we didn't apply it rigidly - for her first walks at 12 weeks, she was probably out for 20 mins, (10 mins lead walking, and 10 mins off lead when we got there) but then we didn't increase this until she was 5 months... She also did puppy romp, which was 1 hr off lead - but the important thing here was that they were all puppies, so there was no pressure from anything else to keep going - and they did rest when needbe.
We all have to make the decisions we are happy with, but I sincerely hope you are never in the position that Maggie is talking about, having to look back and consider whether anything you have done has caused a health issue...