The frequency of your workouts is highly dependent on your goals and what you need to accomplish.
General Population: To stay healthy three days a week of strength training & conditioning is sufficient.
Weight loss: Up to seven days a week. Keeping your body in motion is the focus here. Total body training combined with 25 - 45 minutes of low to high intensity cardio will help you reduce fat, build lean muscle and boost your metabolism.
Muscle Gain: Three to six days a week of hypertrophy training with mild low intensity cardio.
The amount of days is really dependent on your goals and how far you are trying to push yourself. For example, a professional athlete can train up to fourteen times a week. But no matter what your goal is, always remember that proper hydration, adequate rest and a balanced meal plan will be your keys to success!