I did a brief stint of carnivore (beef, salt, water) and one day I realized that I was in a fantastic mood, euphoric almost. I wish I had taken notes but I think it was about 3 weeks in.
Soon after I added things in and have been pretty loose for a few years, though one thing I didn't ever add back in was green veggies.
(Going back to carnivore now! I'm very curious about whether I get that same euphoria again.)