Iggy has got the rite idea. Make sure that when performing cv work that the intensity you perform it at is moderate but for greater durations (20mins plus)This is because fats are a more complex molecule that need to be broken down(combusted) via a a longer energy pathway. For this combustion to occur, there needs to be a abundance of O2 supplied. If the performer is performing at to high an intensty then the O2 is needed else where and at a faster rate.
i hope this helps and isnt confusing. (oh yeh, i wouldn't worry to much about the time just get enough cv in)