Per walk. Assuming there is a reasonable recovery time in between. Of course, this is the maximum not the minimum. If you take it literally then the 15 year old should be walked for 15 hours a day!!!
So yes, at 11 months old the maximum would be around an hour although again, no rule of thumb mesurment is perfect and by now you are getting near to the stage of saying there no longer is a limit. Bone growth having all but finished you are drawing out of the dangerous phase.
Regards, John