Technology research is not measured in turns, but rather the number of research points (the green flasks) generated by your civilization. Each technology requires a specific amount of research points to be generated for it to be discovered.
The file CIV4TechInfos.xml in Assets/XML/Technologies/ directory contains an entry for every single technology in the game. Each entry has a key titled iCost, which contains the number of research points that must be generated for it to be researched completely. For example, Mysticism costs 50 points to research, while Robotics costs 5000.
These values are not the final values -- they get modified by certain factors, for example difficulty level -- but they do represent the base cost of research.
