Time Spent Exercising and Health Benefits

My doctor told me that adding an exercise regimen that kept my heart rate accelerated 30-40 minutes almost every day would be good for me. Doing that (and changing eating habits) helped me lose weight. My question is: is 60 minutes of cardio-exercise ?twice as good? as 30 minutes? Is 45 minutes 50% better for me than 30? Or is there a diminishing return on incremental increases like that?