First off I haven't taken my checkride yet, but I've been signed off and am waiting around for the plane to return from maintenance. That said I logged about 20 hours of ground school during my CFI training. 10 hours over the FOI stuff and 10 hours reviewing FAR's, teaching maneuvers, etc. I spent about 30 hours in flight mastering the right seat, most of that solo and the rest giving mock lessons to my instructor.
During your CFI training, you get to a point where you're almost at the same level of understanding as your instructor regarding the FAR's and aerodynamics. Continuing to pay for ground school at that point has rapidly diminishing returns.
Find the people who had the same examiner you're going to have for your initial CFI checkride and pay those people for a few hours of ground. Since just about any question is fair game on the CFI oral is fair game, it really helps to know what specfic questions outside of the PTS that your examiner is fond of asking.
For instance, I found out that I need to know a lot about the equations for lift and drag, coefficients of lift and drag, etc for my oral. Unless I had spoken with people who had my examiner before I did, there's no way I would be able to explain, much less teach those topics during my CFI oral. I also heard that he failed someone for not being able to say from memory the diameter of the prop on the plane he was flying. I'm making sure to memorize that spec....