It really depends on the dogs. Layla is the same age as Johnie, but Layla is a Boxer. I'd think a Lab mix like Johnie would have more energy than her. But my dogs go for a walk in the morning, roller blading full speed for about an hour, and then a walk or another roller blade ride that night.
But once we move down to the country, we'll have four Labradors and a Boxer, so they won't be getting walks, but more running, biking along, and chasing tennis balls and deer
It varies from dog to dog, but the minimum should be forty-five minutes a day, preferably a couple walks a day since the dog will regain his or her energy quickly after the walk.