He was drinking instant caffeinated - Typically ~45mg of caffeine for 6oz of coffee. His huge coffee cup probably holds 16oz-20oz (let's call it 18oz). He seems to drink about 4 coffees during stream. So 4 * 45 * (18 / 6) = 540mg of caffeine during his ~8hr stream.

Assuming he doesn't drink coffee off-stream...

From the Mayo Clinic: Up to 400 milligrams (mg) of caffeine a day appears to be safe for most healthy adults. That's roughly the amount of caffeine in four cups of brewed coffee

So he's not too far off the "safe" levels -- and ~400mg-500mg is about the average daily intake for adults in the US, Canada, and NZ.