I get this question all the time. The answer depends on each person.
Obviously any physical activity is good to do everyday. But, for an actual workout routine, I suggest 3-6 days a week. How often can also depend on the intensity of your workouts. As well as how fast your body can recover.
I workout 6 days a week. My workouts are pretty intense. My body recovers pretty quickly. But there are times when I am completely wiped the next day after a intense workout. When that happens I will usually use that day as a rest day or do a lighter workout like an ab workout or some sort of mat work like Yoga or Pilates.
If you are trying to reach a certain fitness or weighloss goal 3 days of light cardio and light weights may not be enough. Remember in order for our bodies to change we need to challenge them. Rule of thumb is you should change up your workout at least every 4-6 weeks. Our bodies adapt quickly. I often do the same kind of exercises but by adding more resistance, different interval times or changing up the reps and sets, my body is constantly being challenged.
For me, my workout schedule is always changing. I have to fit it in when I can. As long as you are challenging yourself and keeping a clean diet. You will see results.
My Workout Schedule:
Monday: Legs and Biceps/Triceps
Tuesday: HIIT cardio and abs
Wednesday: Back, Chest and abs
Thursday: Rest Day
Friday: Legs and Shoulders
Sunday: Abs and Pilates/mat and band work.
I will be posting a video of one of these workouts on my website very soon!