Good question. I just assumed 3 full calendar years. I'm pretty sure that the 'birthdate' is when Notch went public with the project and people started being able to play it, but I don't know if that's the same date that people could pay for it.
It had multiple price points so it's hard to estimate profit. People who bought the game early (like me) paid significantly less than the current $26 price.
There are a few historical graphs showing the sales (the wiki keeps track of the various price points too), however I'm yet to find a graph for the whole 3 year period.
Heh, that screenshot is from my website. The utility I built that used to track the sales (during the period of that screenshot) was turned off once minecraft.net became unreliable (used to crash and I didn't have things in place to avoid data corruption). I wish I had kept it going, would have been great to have it still.
It had multiple price points so it's hard to estimate profit. People who bought the game early (like me) paid significantly less than the current $26 price.