My vet advised me ots nor an official thi g at all.
However it is good advice and applies to onlead walking. He says if (at 3 months) its 5-7 mins to a field, then mainly off lead time and 5-7 mins back its fine. Just watch for signs the dog is getting tired when off lead and head back. But just start at a few minutes in the field and work your way up.
They did agree that it protects joints, but it's forced walks like onn lead walls, stairs, jumping in on and off stuff that's worse.
I have spaniels. We have always been able to survive the first year, using this as a guide.
Also, exercise doesn't really tire alot of dogs out. Or not in a 'tired for the rest of the day' way.
I have working spaniels and they don't need as much walking as everybody says they do. They need brain games and a walk where they are off lead alot. Walking a dog alot often ends with a fit dog who is still bored and hyperactive. Or over tired and naughty.
Also tons if new owners are just stupid. We just fostered a spaniel that got lost on a run. It was 6 months old in a 9 mile run with its owner and just gave up and laid down. The owner no longer wanted him because he couldn't keep up and he wasn't will to cut back his own runs until the dog was one and to work his dog up to it.
I see so many spaniel owners complaining their dogs are under 1 and still hyper despite 2-3 hours a day they don't understand that won't work and ictially makes them over tired and worse behaved.
I have always wondered if its slightly exaggerated because so many people think dogs can walk miles from a very young age.