
The rule I followed was a bit different though I'm sure I got it on this forum.
5 minutes of forced exercise/per month of age, allowed twice a day. Forced being anything on leash or where pup is obliged to keep up a straight line of movement or is on a hard surface.
Free time off leash on ground or grass unlimited as pup can stop, change direction and rest as he needs.
I followed this loosely. My Lab was mostly off leash but had to go with me along the trail in one direction due to deep snow off the trail. And I never took him out on leash on a hard road for as much as one hour in the morning and then another hour at night, which is what would have been allowed with this rule by 12 months. We did go off leash and by the time he was 12 months deep snow was not a concern. I would not have allowed a total of two hours a day, divided, if he was playing with other dogs either.
But if all he got was 40 minutes a day by 12 months I really believe he would have gone nuts and so would I. I kept an eye on the time till around 18 months old too. Also an eye on the terrain. And the cold since he was a winter puppy. Oh, and better add, deep snow was a concern at 14 months; tons and tons of it, and the breeder specifically warned against too much bounding through deep snow. We didn't get into serious snowshoeing till his third winter. Snow over his back meant I cut that outing shorter too.