Excessive sweating, even excessive face sweating, is your body's natural response to the increased energy expenditure of exercise. Your body is always trying to maintain a core temperature of approximately 98.6 degrees. When this temperature is elevated, your body sweats to keep you cool. What you perceive as "excessive" sweating is probably normal. The range of "normal" sweating can vary as much as three liters from person to person, so while you might feel like you have an excessively sweaty face compared to others, it is still probably normal.
If you are exercising (especially in a gym), you are going to sweat a lot. Instead of worrying about having an "excessively" sweaty face, try carrying around a towel and/or wearing a sweat band. If you are new to exercising, you might feel like you are sweating more than others; maybe even 10 minutes on the treadmill makes your face excessively sweaty. This will decrease some as you become accustomed to exercise.
As previously stated, it is difficult to quantify an amount of sweat as normal or abnormal. What you should consider is when you sweat. If you only sweat excessively when you are exercising, than there is little cause for concern. After you have cooled down from your exercise, your face and the rest of your body should stop sweating. If, however, you are sweating while you are watching television or sitting at your desk, you should see a doctor. This is especially true if you are experiencing other symptoms such as night sweats, shortness of breath or weight loss. These could be symptoms of a condition (like diabetes) that needs medical attention.