I'll try to answer the "time" aspect of a commerce degree. From my experience so far commerce degrees have a lot less contact hours than say a science or engineering degree. However that isn't to say they are less time consuming. I know people who have studied subjects from all kinds of disciplines and they say commerce tends to have a fair bit of recommended reading. As with everything at uni you're going to get out what you put in. Therefore I would suggest doing all recommended readings and tutorial questions required of you, as long as any additional questions etc provided by the lecturer, personally I find I probably double the "contact" hours for the subject by the time i've completed everything. Doing all this and of course understanding it will set you up well come exam time.