First thing each day (about 7:00am)we go out, they do business and we walk 1/2 mile after business.
Approx. 10:00am, we go out again, and we either go to the park on the weekends for a romp with each other or with their doggie friends, for about an hour. They play full out, non stop.
After lunch, we walk another 1/2 mile and come home.
Before dinner, we walk another 1/2 mile.
About an hour after dinner, we take a 2 mile walk, or circumstances permitting, we go to the park again and they play for another hour.
We go out for another 1/2 mile walk around 9:00pm and that's it for the day.
Of course, we can't always meet this schedule, and we do what we can on bad weather days. On those days, they tend to play in the house with each other, fetch, chase, wrestle on the bed, etc.
This seems plenty for my two girls. They are tired and we come home and they most always lie down and rest or sleep for at least a couple of hours after playing in the park. They are not destructive in the least when I'm not home because they have received plenty of exercise and know that they'll get more fun and play time when I get home. So plenty of exercise has, in my opinion, been one of the major reasons why my dogs behave well at home alone, they don't have sep. anxiety issues, and they are extremely fit and muscular and healthy as well.
Remember also that mental exercise such as training and working on heeling, recall, etc., etc., is also as good as physical exercise at wearing out your dogs. If the weather is bad, we will do some trainnig exercises in the house for about 1/2 hour, then play some and do quick outings for "business" only. This works well for me and the dogs too.
Good luck