100mg would be a pretty big portion. Maybe a grande from Starbucks (with notoriously high caffeine content) is about that. Isn't a normal cup about 40mg?
Well, it varies so much, I just picked 100mg as a nice round number. However, after a little more research I found this website [http://www.energyfiend.com/the-caffeine-database] and did an calculation on the average mg per serving and it came out to 109.97, so I think 100mg was a good estimation.
