Until now, it was the general rule that you need to do at least half an hour condition training in order to start burning fat. Now, shorter but tougher workouts are the trend in 2016. The pulse goes up shortly before the maximum. High Intensity Training is the trend now which means intensive workout in a short period of time. This is especially interesting for everybody with little free time in daily life.
Personally, I go to High Intensity Cycling (HIC) every week. In half an hour we drive a couple of intervals with high resistance. After half an hour I am truly exhausted and still burn calories in the coming hours.
The same principle is of course not only working for condition training but also for strength. With more weight but shorter units you can also make a High Intensity Training work. But I would be more cautious here. At the end of the day sports should be good for the body and with too much weight, chances are high that the result of the workout will be rather painful in the coming days.