3-day fasts are a sweet spot for me. When I consider going longer, it’s all about how I feel at the 3-day mark. If I still feel pretty good, and I have the flexibility in my schedule, I’ll push further.
I don’t see a problem attempting a 5-day fast without having done something longer than a 24. Just listen to your body, make sure you’re drinking enough water and getting your electrolytes. If you’re feeling it, go for it.
Best of luck!!