How fast can CPU governors change their frequency?

All CPU governors descriptions that I could find use subjective words such as slow and fast to explain how the frequency changes over certain conditions. What are the typical values here, are we taking about over or under 1ms? I'm curious about a) how fast can the hardware go from min to max and vice versa and b) what would be a reasonably conservative number such as in a on demand cool down? Are there any typical values for say an open source ROM or any other, in such and such device, available anywhere? Or in the lack of it, is it feasible to measure my own device over adb/ndk/sdk tinkering?