This will vary according to the type of exercise and the person's body weight. If we take a person weighing 68 kg (about 10 stone 10) as an example, the calories burned in an hour of moderate exercise would be around 175 to 300 depending on the type of exercise. For example, cycling at 5mph or very gentle jogging would keep you under 200 calories per hour, while swimming and tennis, even if done quite slowly, would be nearer 300. It's also good to know that even light housework burns roughly as many calories as recreational horse-riding, while if you have a really energetic spring clean it can be similar to an aerobic workout!
Faster jogging or cycling, or tough sports like cross-country skiiing will raise the calorie burning much higher - 600-700 per hour. You can see more exact details here, and also if you choose a sport, type it in a search engine and add 'calories burned per hour' you can a precise answer.