Is there a point at which additional exercise doesn't help any more?

Many benefits. You get continued benefits from exercise, metabolic changes, calorie expenditure, improved mental health from any and all exercise. Most of these benefits can be accrued with less than an hour of moderate intensity exercise/ day. Some can be gained with less like an hour or two a week. There are still some benefits such as increased fitness and weight control with more than an hour a day.
Not really. In general, the benefits of exercise are limitless. One can always improve their level of fitness. Sometimes, people can plateau with long-term continuation of the same exercise routine. Modifying exercise routines by varying intensity, exercises, exercise environment and timings and by cross-training and taking a week-long muscle rest every 6 weeks can help you see continual improvement.