I think most people would agree with a GA right after you are out of despotism. There may be some exceptions, but the basic reason is that Civ3 is a game of exponential growth. 1 city becomes 2, 2 become 4, 4 become 8, etc. This growth is eventually blunted by corruption, though generally the more land you possess, the better positioned you are to compete with the AI. In addition, the earlier you assert dominance, the easier the game will be.
Despotism usually takes too much of a toll on the benefits of the GA though sometimes a Despotic GA isn't so bad.
I think that there are two basic schools of thought - one is to wait until your core is stable so you can reap the benefits of an established, powerful core. The down side is that you need to hold of the GA until you are ready. The other is that the GA will help you establish such a core much faster and thus out produce the civ that is still 'preparing' to move out. The down side is that the multiplier buildings may not be around to capitalize off your GA.
That can be a tough crystal ball to gaze in to.
An early GA could help you conquer a neighboring civ and doubling your size. A later GA may help you push through the early Middle Age tech trap. I have rarely found a GA in the IA or MA to be of much use. Even late Middle Ages seems to be pushing it.