First of all, fasted training is outdated. It brings more harm than benefits.
When I’m completely running on fat and have used up all my carbs then I hit a fat burning wall. It’s around 5 bpm lower than my AeT depending on the sport.
Yes, you can burn glycogen when fasted since you have around 500g of glycogen in your muscles and 100g in your liver. So let’s asume, you burn 60g of carbs per hour and the glycogen storages are 100% full (which the are not after fasting) then you can go around 6h burning glycogen.