There are 4 factors in aerobic training: Type of exercise, frequency (# of times weekly), duration (length of each workout), and intensity (how hard you work).
1. The type of exercise you select is important because it must be something you enjoy or at least tolerate regularly. The more you enjoy it, the more you will be motivated to do it! There are endless forms of aerobic exercise, including jogging, walking, cycling, aerobics, sports, cardio equipment, swimming, etc. If you are training for a sport or race, be sure to include sport-specific cardio workouts. It is good to include different types of exercise if possible (for example: walking and cycling).
2. The frequency of exercise depends on what you are training for. If you are starting an exercise program for the first time, you may not be training for any sport or race, but setting your frequency ahead will help you stick to your regimen. Perform your aerobic exercise at least 3-4x/week (or more). Take a rest day after a very difficult, long, or extra-challenging workout.
3. The duration of your aerobic exercise depends on the intensity (#4). If you exercise at a higher intensity, you can work out for less time per session (effective if you are super busy and have only 20-30 minutes/day to exercise). Likewise, a low-intensity workout means you need to exercise longer in each session.
4. The intensity of your exercise can be easily measured by checking your exercise heart rate (in beats per minute). Pending your doctor’s recommendation, you should exercise at ~60-80% of your maximum heart rate (220 minus your age). If your intensity is too low, you will not get the most benefit from your aerobic workouts. Be sure to increase your intensity as your aerobic endurance increases (for example, increase average pace from 3.0 miles per hour during week 1 to 3.5 miles per hour during week 2).
Baechle TR, Earle, RW (editors). Essentials of Strength Training and Conditioning, 2nd ed. 2000: Human Kinetics. Pages 498-501 (Chapter 21: Potteiger, Jeffrey A. Aerobic Endurance Exercise Training).