In reply to arch:
Obviously the 'correct' amount will be dependent on the age/breed/temperament of the dog.
We've got a two year old border collie, so obviously he needs a lot of exercise. I run almost every day through the local forest trails with him off the lead for about an hour.
He also gets another 30-40 minute walk off the lead after work.
Both me and my wife are lucky enough to mostly work from home, so several times a day we'll take him out into our back field and spend 10 minutes throwing a frisbee, which he adores.
It's not just the exercise (especially with Collies), it's the mental stimulation and the feeling of 'doing a job' in the pack. The morning run/walk is important as it's meant to simulate the daily territorial roaming instinct in dogs. Although we've got lots of land, we can't just use leaving him free to run around as an excuse not to engage with the dog.
It's all topped off with a before-bed 10 minute walk for a pee.