I'm no scientist, but I've been in the coffee industry for a long time. I would say more than 500mg per day is a lot. 200-300mg per day seems to be a nice sweet spot. That's about 12-16oz of brewed specialty coffee.

Once again, I'm not a food scientist, but I believe other ingredients you find in energy drinks can compound the effects of caffeine. Similar to how alcohol mixed with certain medications with fuck up your liver really fast.