Officially, EASA thinks that excessive consumption is more than 250 mg per day. The usual reference for HPL is a huge book which I don't have with me, but I will look at my earliest opportunity. However, given that most human factors questions were extracted from the newspapers, who knows?
As a matter of interest, when I joined the Army, they showed us a movie of someone who had been injected with a week's worth of caffeine in on hit - he was walking around like a drunk. I rather think the intensity might have had something to do with it.