At one time, I could not do this with a 30 pound dumbbell in my hand. I started out with no weights.
When I train I don't usually focus on how much calorie or fat I burn. Furthermore, if I'm not training for anything specific, such as a show or a shoot, I don't worry about "how long" my workout should be.
Depending on how I feel that day or how much time I have, sometimes my workouts are 10 minutes, sometimes 20, 30 or 40 minutes. I train with a focus on improved strength, performance and health, not too dependent on time or calorie count.
Hence, I create and include a lot of different strength challenges into my workouts such as this renegade row from push-up to side plank with 30lbs dumbbell.
Whether to lean up or change your body composition, if you want to achieve real sustainable results that last a life time, do not workout with the sole purpose of calorie burn or fat burn. Focus on strength-building instead. Training for strength is way more fun—and when you're well-fed, it can be far more effective at helping you stay lean than training solely for fat-loss.
No need to stress, just know that if you're getting stronger and your performance is improving, your body will burn, grow, sculpt and ship-shape itself to match the way you challenge it. Then, if you want to see more results, you can start honing down on very specific workout programs to further your goal. But don't be surprised, you might just love the way you look and feel at that point and won't feel the need for more. ;)