Duration
First think about quality versus quantity. If you are going really slow and using very little effort and you are on the treadmill for two hours or running up a hill for sprints, which one of those exercise techniques do you think will work best for you to lose weight?
I am a firm believer in what you put in is what you will get out (as long as it is smart and calculated). The more effort that you put into your cardio, the better and faster your results you will be.
Like any other workout program, you should never start at an advanced level. For example, if you have not worked out in years, then you should not start out with high intensity incline sprints. This will be too much intensity too fast and will cause an injury.
But if you have been working out consistently (3-5 times per week) then you should definitely consider increasing your intensity and pushing yourself as hard as you can without getting injured.
So you are probably wondering how long you should do cardio to get the best results. I suggest that you focus on doing cardio sessions for twenty to thirty minutes at the highest intensity you can handle each session.
Each week try to push yourself a little more by increasing your incline or speed of your workouts based on what tools and exercises you are using to get your cardio work done.
Treadmill Versus Bike Versus Running Outside
The best cardio workouts are those that you enjoy, understand, and will stay most consistent with. If you love swimming or mountain biking ,then choose those forms of activities to measure and use as your main source of cardio work.
You always want to switch up your workouts so your body does not plateau, but you have to have a main source of cardio that you can go to to get results.
In my opinion, there is no one source of cardio better than the other. This is a common question that I get in the gym all the time by people who want to lose weight.
My simple suggestion is, they are all great ways to lose weight so choose the ones you enjoy and do it consistently.
Heart Rate
I would highly recommend that you get a heart rate monitor to track your effort and intensity in your workouts. Tracking heart rate is a great way to measure your exercise progress and can give you important data on how you should train.
The best data that it will give you is if your heart rate has been one hundred and thirty eight every single workout for the last four weeks and you have not seen results, then that will tell you that you are not pushing yourself enough to break your plateau.
The next step would be to either start running on an incline or a hill or push yourself to pedal faster in your workouts, or swim faster.
Your heart rate can be a great number to use to guide your workouts and take you to the next level.

Author's Bio: 

Fiture was founded by me, Greg Marshall. I understand that people want to live the best life possible and I firmly believe that health is the most important thing in the world because without health you cannot reach your potential in all of life's aspects without it.

I have been in the fitness industry for close to a decade now and have ran several personal training businesses, helped thousands of clients get involved in fitness regimens, and I want to change as many people's lives as possible. Fitness and personal training is my vehicle for achieving this goal and touching as many people's lives as I can.

I sell fitness because I believe in it 100% and know how it can change a person's life in an instance. Fitness gives you an opportunity to prove to yourself that you CAN do anything that you set your mind to. Sometimes all it takes is someone to give you the knowledge and believe in you to help take your life and happiness to the next level.

I currently run a personal training organization out of Salt Lake City, Utah at The Gym at City Creek. I have an amazing team there who helps me increase my knowledge, motivation, and truly gives me an opportunity to positively affect more people than I can even handle.

My goal is to help you change your life TODAY!