Recent studies have demonstrated that it may be possible to achieve more weight loss with shorter exercise sessions. The studies compared equivalent workout levels (each burning 600 calories per hour) with different lengths of workouts (one group worked out for 30 minutes and the other, 60 minutes). Normally, we associate more calories burned with more weight loss. In this case, the opposite happened: the 30 minutes group lost 20% more weight than the 60 minute group. One suggestion as to why this may have been the case is that the group with the shorter workout tended to be more active throughout the day.
They also considered the fact that the 60 minute group simply had less energy after their workouts, and therefore their cumulative calorie burn over the course of the day would be less. In my opinion, this is probably the most likely explanation. Fat storage is zero sum: you can only store from what you have extra. If you eat fewer calories than what you burn, then you burn up energy from fat storage. It is possible to break down protein for immediate energy use, and this can happen in endurance runners for example, but in the average person, this is unlikely.
My suggestion is to work out as much as you can without the feeling of being lazy after the workout. Keep your activity level up throughout the day. Take the stairs, walk instead of drive to a nearby destination, or get off of a train station a stop or two earlier before heading in to work.
Stay tuned to more 30 minute workouts to be posted. Check out some of my previous fifteen minute workouts here or here