I think with a lab, I would reccomend at least an hour of exercise a day due to high energy level, of course that doesent mean letting the dog run around the backyard for an hour, mental stimulation is very important, regular walks, playing with other dogs, running off leash will keep the dog in great shape.
I have a 67 lb , 4 year old weimaraner, we do a 4 mile jog in the morning, then I get home at 5 and let him off leash to play with neighbors dog for 15-20 mins, then we go for another 2 mile walk in the evening.
On the weekends we usually go on at least a 5 mile hike, and I let him run around and play with other dogs at the dog park for at least an hour, depending on temperature. In the summer, I pack up my kayak and we head out to the lake, he loves to accompany me on my kayaking trips and then I let him swim around to his hearts content.
But keeping my dog fit and healthy is not only beneficial to his health but to mine as well, I would be a total couch potato if I didnt have a dog.