Finnish people drink a lot of coffee; in my place of work it's a mug (0.2 litres) every two hours or so during a twelve hour shift. Some people there have been doing that for over a decade and apparently their dosage and consumption interval has remained the same, keeping blood caffeine levels quite steady.

Finland has the highest coffee consumption per capita in the world with ~12kg annually; compare to the US with ~4.1kg: http://www.worldmapper.org/posters/worldmapper_1038_coffee_c...