I've experimented with this before, and found that my caffeine-assisted productivity hit its highest levels when I had a cup of coffee every three days or so. More frequently and I built up too much tolerance; less frequently and the stimulant effects were too powerful when I did drink it, giving me distractibility and a tendency to fixate that weren't made up for in alertness and mental agility.

Your body chemistry might vary, of course. Also, that's peak, not sustained, productivity; you're probably better off with a steady intake of caffeine if whatever you have to do will take more than a day or two.