how often should I do a resistance workout and how long should it be?

I'm at about 195-200 lbs at the moment (16 years old) and am trying to lose bodyfat to become leaner. I went through scooby's website and read everything that applied to me and have been able to implement eating healthier, drinking more water and getting more sleep. I am a full time student and get a lot of homework and studies to do everyday therefore i am only able to fit in a 30 min. cardio session everyday (I do a 30 min. swim). I've been doing this for about two-three months now. One thing I am severely lacking in is fitting in a resistance workout. The reason for this is because I am confused about the following things:

1) How long should my resistance workout be so that it is effective?

2) How often should I do a resistance workout for it to be effective?

Thank you very much, all help is appreciated

1) Around 45 minutes or so, but it depends on your workout.
2) Around 3 times a week. Make sure to have days of rest in between workout days.
3) When you stop getting gains.