How much exercise is too much?

Exhaustion. When you're uncomfortably short of breath or exhausted, you've exercised to your limit - there's no harm in that at your age but you've found your personal limit. With continued workouts, your limit will be stretched out further. That conditioning process takes weeks to months (more than "days").
Difficult to say. In an adult, excessive exersise which causes symptoms of dehydration or overuse tendinitis of the knees or other joints despite being in excellent " shape" is probably ' too much'.Check with your physician before u start an ambitious exersise program or increase its length or intensity... !