It depends what you are doing though. An exercise that only works cardio might raise the metabolism whilst you are doing it, but fall again once you have finished, creating more of a 'spike' in your routine; whereas doing weights for the same amount of time has an affect on your metabolism that can burn up to several days later, putting it in overdrive. I've found it effective to have a weights session on a friday afternoon, (being that i don't do anything on the weekend except drink and eat), because it burns all weekend. The best workouts are the ones that are part of the lifestyle, and is why the hard tradies are usually pretty fit, because they are working for half the day, which is how we were built. So, effectively 2 hours could be useless for too much bodies but great for others. I personally think that 1-1.5hr of mixed weights and cardio is plenty for results for most people, but if you really wanted to get the metabolism burning all day, maybe 2 or 3 sessions a day, short and sweet, with maybe weights in the morning to really kick start the metabolism and then a fat burning exercise at night to help get rid of unwanted energy stores. I've found personally that endurance and weight loss aren't necessarily the same thing, and that really, a lot of people are different and need different approaches. Unfortunately, most people can't spare 2 hours a day to exercise though.