“30 minutes a day” is the advice you’ll see everywhere, but it’s a rough average that ignores enormous differences between breeds, ages, and individual dogs.
A working breed like a border collie or vizsla can need well over an hour of real activity, not just a stroll, to stay mentally settled. Brachycephalic breeds like bulldogs need much shorter, cooler sessions regardless of how much energy they seem to have. Senior dogs often do better with two or three shorter walks than one long one.
Mental exercise counts too — a 15-minute training session or puzzle feeder can tire a dog out as effectively as a jog. If your dog is destructive, restless, or barking excessively, it’s worth treating that as an exercise question before anything else.