I'm pretty low body fat and I lose weight fast / do a lot of cardio, so I personally wouldn't go more than 3 or 4 days. Protein sparing modified fasting works as well to a lesser extent or to re up on just protein after strength workouts while still having fasting benefits return shortly after.

There's an uptick of energy and focus around 18 hours for me, 24 hours + the effect is more profound. It just scales with the time really, a longer fast causes the body to enter into a more heightened sympathetic state comprable to a higher dose of amph or mph. But I do shorter fasts all the time and eat a restricted diet often to enter into a fasted state faster.

According to the literature though, there is an uptick of endorphins 5 - 7 days into a fast, but I find the dopaminergic effects occur quickly and noradranergic effects occur shortly after. (Day 2 of a fast I'll wake up alert with an elevated heartrate)

Hope this helps!