How many times a week should an individual exercise?

As Wendy said, it depends on the goals. As a general rule of thumb I recommend the following to most clients:
3-5 days of cardio
2-3 days of weight training
1-2 days of stretching

Stretching can be incorporated into each and every workout, and does not have to be designated to just 1 or 2 days of the week. Remember the workouts can be combined to have elements of all three types of workouts. As always, work smarter, not harder. There is a fine line between working out in a safe/healthy manner and over training.