Chen Taijiquan

Aerobic means "with oxygen" or "in the presence of oxygen." It is any training which rhythmically uses large muscle groups. Aerobic training increases the heart, lungs and cardiovascular system's ability to carry more oxygen quickly and efficiently to the body. As the heart muscle becomes stronger, it will be able to pump a larger amount of blood with each stroke, hence, decreasing the number of strokes needed for oxygen transport. An individual who is aerobically fit can train longer and recover faster. 

This will depend on what you want to archeive and your current physical state of health. If you are a beginner and recover from illness, moderating is important.
If you are after a healthy life style then it is commonly agreed amongst experts that 3-5 times per week is sufficient. You should be exercising for at least 20-60 minutes at 60-90% of age-specific maximal heartrate or 50-85% of VO2max (heart rate reserve). However, remember that your body needs time to repair. Your body won't rebuild until fater your training workout; no rest, no rebuild.