there is not a certain time just reach, 60% max Heart rate as wardy says. The time I think you are thinking of is 12 minutes at 80% max heart rate for any cardio improvement.
I work in a gym and was working out numbers showing some people they were better off training at a higher range. I know the ratio for the fat burning range is 50/50, carbs to fat but i also know you don't get that as soon as you get above 60%MHR. I assume it varies from person to person.