Most new exercisers respond well to three full-body weight-training sessions a week, with three to five sessions of cardio / aerobic training a week.  This might sound like a lot, but the cardio sessions (walking, jogging, treadmill, elliptical trainer, exercise bike or stair climber) should be 20 - 30 minutes, and the weight training sessions shouldn't take much longer.  Though it's important to challenge yourself to avoid boredom, it's as important to avoid overtraining and burnout.

At the end of a workout, you should feel pleasantly tired and pumped, not wiped out.  Feeling good helps your motivation so that you'll always look forward to exercising.

As you make progress, you'll feel like adding to the challenge.  You can increase the length or intensity of your cardio training (helping your heart and lung capacity and burning off more calories) and add resistance and new exercises for body parts you'd like to target for special attention.  An old bodybuilder's secret is to work your target areas first, when you're fresh and raring to go (Weider calls this the "Priority Training Principle").

Eventually, when you feel you've moved beyond the beginning stage, you may decide to switch to a "split" routine where you train four days:  alternating half your body parts one training day, and working the rest on the next day.  This keeps your workout from getting too long and unwieldy, and also gives each body part a longer recovery period to rest and grow between workouts.  I like the "push-pull" split, where you exercise body parts that work together in the same workouts.  The muscle groups that push, the shoulders, chest and triceps (back of the upper arm) work one day, and the pulling muscles, the biceps and back, work out the next day along with legs.

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