Using SC2gears and some Excel kung-fu, I tried to answer these questions and more by looking at race win rates based on the length of the game from all games at MLG Providence. I came up with this set of charts showing what happens as the game progresses (click on the image for all the charts).
Data here: MLG Providence Race/Length of Game Win Rates Note: times here use in-game time, not wall clock time. 5 minutes of wall-clock time is about 7 minutes of game-time
The data does show some interesting results. But I'm not satisfied that the sample size is large enough. I'd love to repeat this analysis on a bigger dataset if there is fresher and larger replay pack lying around (coughplayhemcough).
To the Pros, Masters players, and everyone: do the numbers here line up with how you think about the game? Is Terran overmatched in the late game? Does Zerg grow stronger as the game goes on? Should you 6-pool (100% Z win rate in PvZ <5 minute games) or 2-rax every (80% T win rate in ZvT 5-10 minutes) every game? Discuss.