OK, this UA article says about 12 hours:
“When you awake in the morning you will have presumably not eaten for about 12 hours. Your glycogen stores will already be somewhat depleted, even if you did not engage in a heavy training session the day before (more so if you did). Glycogen depletion has been shown in numerous studies to be one of the most powerful aerobic adaptation stimuli by signaling an increase in aerobic enzyme activity. So starting a workout in a semi depleted state kick starts the fat-as-fuel-adaptation process….”
So I guess I will just need to experiment with 6-8 hours (i.e., lunch, evening workout, late dinner) and see how it goes and how I respond.
Thanks for any feedback on whether shorter times than 12 hours still carry some of the benefits!