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.

Coffee doesn't seem to really help me aside from keeping me from falling asleep for a short time (~40-60 minutes maybe). I used it most heavily (2-4 small or medium cups a week) when I was suffering from undiagnosed sleep apnea, but even now I occasionally have some when I feel I must be extra sure not to get sleepy, and it doesn't really seem to help me focus and wears off quickly. Does coffee not work on certain people or is there some factor I'm not aware of?