45 to 60 mins duration will always be enough so long as the intensity is correct.
what is the unit of intensity? HR max, VO2 max, lactate max? the essential component to weight loss is not the cardio side but the diet aspect. assuming that you have a healthy diet then working at 60-70% of your VO2 max would be perfect for keeping trim.
hope this helps
Daniel
well i'm an avge runner so my VO2 isn't that great. My diet is somewhat clean and balanced.