This is probably more relevant for Protoss and Terran players, and a similar Zerg question can be found here.
Obviously each build order is different, but you never see build orders which take you past 40-50 supply (and that's on the high side already). For instance, how long should you sit on 2-gates or 3-barracks prior to adding new ones when your build order is "2-gate" or "3-Rax" respectively?
In my own games I find that although I can execute the openers well, once I get out past 40-50 supply, I find myself adding more Gates or Robotics Facilities, because I want to get more units now, but I often wonder if sitting on 2-gates (just as an example) longer would result in a larger army and a stronger push because I would have more minerals/gas for units.
So, certainly unit composition being produced will have a significant effect, but what is the average number of Tier 1/Tier 2/Tier 3 buildings each base can support with saturated mineral/gas lines, and what is an indicator that I have too few of any of the tier 1/tier 2/tier 3 buildings and should add more when transitioning past the standard race specific build orders?