As a general rule of thumb, I will settle GS until I have Labs. The time between Public Schools and Labs is iffy depending on my current tech rate and goals for the game.
I generally do not use
to purchase GS until I am ready for the end game bulb fest. By the time you can buy great people with
, and can also start to stockpile
as the automatic great Prophets stop consuming your
in the Industrial Era.
I find that for a good Science Victory, I will plant early GS up until the Renaissance/early Industrial Era. I will get Research Labs and buy (or build if not enough
) in key cities. If timed right, eight turns after I get my Labs should be about the same time that I get the Rationalism finisher to bulb 2 free techs (for which I pick Rocketry and Satellites). At this point I can buy 2-3 GS and 1 GE with
. I will use the GS to bulb techs and the GE to rush Hubble. I will also start building Apollo in my highest
city. When Hubble finishes I use 2 more free GS to bulb techs. By the time Apollo finishes, I should be able to build all the SS components.
For a Diplomatic victory the game plan is pretty much the same, accept the tech path is different. I will bulb 8 turns after I get Labs in an effort to unlock the UN. I will also have filled out Patronage in the early part of the game to ally all the CS which is not as important for a Science Victory.
For Cultural Victory I will generally buy GA with
and a successful Domination game seldom reaches the Industrial Era