It depends on your objectives.
But generally speaking, when starting out, you should work on increasing your duration. Once you get to the point where you can comfortably work out for 30-40 minutes, you are better off adding more intensity, rather than more time.
Also, strength training should be part of any exercise program. Once you get to woking out 30 minutes 3 times per week, your next step should be adding ST to your program, rather than more cardio.