In reply to Auld mick: I think the general idea is that not to overdo it for the sake of their joints, the bigger the dog, the more this is an issue, i also think some dogs like you said are mroe developed and cope better, i didn't do over 6 hours with my springer until he was 12 months, not for any reason, just the way it worked out. Is the dog on or of a lead? May sound a daft question, but on a lead my dog could go for days, off a lead he charges round like a daft goon all day, only shown signs of getting tired after about 9 hours, and in this 9 hours, he is fetching and doing retrieving work constantly!
The one thing ill say, is that if you took him out now which vets would say is 'early' and he developed a serious joint problem in the next couple of years, you would blame your 'impatience' for the problem, which would make you feel pretty harsh?
You and only you know your dog, what its use to etc, young wolves are out with the pack hunting when under 18 months, which puts from more strain on them, than walking with us slow humans!
If your pup has been mollycuddled and never climbed over anything, bumped into anything or jumped of anything etc, like some of these fasion dogs are, then your asking for a disaster, if its a pretty hardy pup, then id have a go, take breaks, and turn back if she/he seems tired?
My input is probably worth nothing, but not all breeds are the same, nor are the dogs within that breed, how can you put a standard time to exercise on them all? A 6 month dog allowed 30mins a day?? Thats way too low, even though it is whats recomended, is that on a lead of as well?