Have you ever been to the dog park or beach and seen a cranky pants? The dog who doesn't want to plays but gets mad easily and is overly aggressive. That particular dog may have that sort of temperament but he might also be over tired. Too many dogs that go to the park or beach stay too long causing exhaustion aggression. I remember back when Luke was little and we were at the dog park often for socializing. When I arrived I regularly saw cranky dogs and when I asked the owner how long they'd been there, some told me hours. Wow.
Dogs are all different, each one very individual and their exercise requirements are the same. But no dog has to run for hours and hours unless of course that is their job. Exercise is a build up to it thing just like with humans. You cannot expect a dog to run 10 miles which is easily done at a dog park in several hours and not be phased mentally by it. There is also a huge difference in running with other dogs and just running. When a dog is at a park with constant canine stimulus it can get to be too much. Interacting with different dogs can be stressful, even for the most social types. Stay there too long and things can go from great to very bad.
So how long is too long. Know your dog. Are they a very social type who deals with lots of different dogs easily? Maybe they are social but only like one on one really nice and easy play. Really knowing your dog and their limits is essential. My Motto for dog beaches or parks is "short and sweet." Get in and get out. Always end on a positive as well, that is very important. If something bad happens at a park, stay long enough for your dog to have a good association before leaving.
Even play dates in your own home must be monitored and kept short. When Elsa and Penny are playing, I often grab Penny and put her down for a nap. Neither will nap on their own when in each others company so I must facilitate a rest time so that it doesn't get too rough and out of control. Penny is still very young and gets tired easily although she'd never admit it. Elsa can go forever I think but she too needs sleep and does much better with lots of time outs. The play gets fast and out of control when Penny gets tired. But before that happens I will pull her out and put her down for a nap.
Dogs can get their fill of stimulus and exercise very quickly. They naturally sleep far more than we do and they most definitely need it. Think of a pack of wolves, most of their energy is spent on a kill. The chase is intense which demands a huge amount of energy reserve. They don't waste a whole lot of unnecessary monkeying about.
Tired is good, exhausted is not. Dogs who are exhausted and continue to interact with others can get very cranky. Just think how you feel at the end of a rough day.
Remember this always when you are out socializing with your dog; "quit while you are ahead." My motto in life.