Is there actually a time window when you start burning fat? Or is it in general the fact that you’re consuming less calories?
My understanding is that ultimately, it depends. How much did someone eat, what did they eat, have they already depleted their glycogen stores, what their activity is like after the meal, etc.
If you want to get really pithy you’re technically always burning some fat, it’s just the rate and what the dominant source of energy that changes. The body is a big complex system, it’s less binary ons/offs than we think it is.
Dr. Jason Fung on the stages of fasting:
https://www.youtube.com/watch?v=T7o_hF1XB84
As I also recall, he’s said 16 hours is really when the good effects are just beginning. That isn’t to say 16 hours everyday isn’t very beneficial as there’s a reason it’s the most popular form of IF/TRF, but occasionally going longer is probably worth.