Not what you want to hear, but… never. The effects of fasting are different for everyone. Some say they feel more energized, I feel no difference in energy if I fast or not, and I’ve been doing OMAD for a long time - have also done 16:8, 18:6 and 20:4, none of which have affected my energy levels.
Again, probably not what you want to hear, but the only thing I noticed really gave me energy was working out. Just 10 mins a day, not an intense workout at all (squats, push ups and crunches), but there’s a huuuuuuge difference from when I wasn’t working out and my energy levels were at like 15% all the time.
So, if you have the time and need energy, I definitely recommend working out regularly, even if it’s just 10 mins a day.
I have a lot more energy now but I attribute that to eating low carb, not intermittent fasting. Though I am surprised at how much energy I have in the mornings before I break my fast - I can usually hit the gym near the end of my 16 or 18 hour fast.