When the human player initiates a revolution, or the government "falls" due to disorder issues in Democracy or Republic, the option to reset the government will only appear on one out of four turns sequentially throughout the game. A guy named Oedo discovered the pattern, which looks like XXXOXXXO. So if you have 50 year turns from 4000BC to 1000BC, the reset turns are 3850BC, 3650BC, 3450BC, and so on. If you order a revolt at the end of 3500BC it will take effect at the beginning of the next turn, 3450BC. But if you order the revolt during 3450BC it will not take effect until 3250BC. The revolt must be ordered on the turn before it takes effect, unless you discover the tech during the city processing phase before an Oedo turn takes effect. If you order the revolt the turn before, your cities will not produce gold or beakers but they will produce shields.