I’ve kept a somewhat careful log of my extender usage. I started with no manuals and gained 0.4 inches in 530 hours of using an extender. From published peer reviewed studies, I have concluded that one can expect to gain 0.1 inches for every 100-125 hours spent in an extender alone.
However, doing manuals first and then strapping into an extender can speed this process up as this is something I have noticed in my own experience.