The right amount of exercise?

We have a 1 year old Lab and she loves to play with other dogs. When she does they tear around and race to and fro, its lovely to watch.

Quite often that will happen when we are out walking and I've been roughly following the 5 minute rule (i.e. 5 minutes exercise per month of life) which should be about an hour's exercise per day but in all honesty I go over the amount a bit at about 15-20 mins in the middle of the day and 40 mins to an hour at night.

Obviously on those days where she has a play with other young dogs she gets considerably more of a work out than those when she doesn't and other than in the recent heat she's not shown any signs of tiredness.

Now I know that its is to protect the joints while they are forming but my thoughts are when does the 5 minute rule plateau?

Also if she's had a big run around when we are only just into a walk would it be advisable to cut the walks shorter.

FWIW she's a small lab (20kg) which I think slightly reduces the risks of arthritis in later years.