As with duration, the frequency of your exercise is often a matter of lifestyle. Most experts feel that the minimum frequency is three to five time weekly, but that depends, of course, on duration. As long as people are careful to avoid injury, there seems to be no maximum limit on frequency of exercise.
In the real world, it's a different matter. For those of us with jobs and family responsibilities, the question of how often to exercise has little to do with sophistry and much to do with the choices we make to balance our lives.
At any rate, three times per week for 30 minutes is a BARE minimum for meaningful health improvement and for weight loss, twice that at least.