The first thing to understand is that the flight instructor certificate training and checkride is not about flying. It is about teaching. If you're reasonably proficient at flying, you'll need little time in an airplane practicing the maneuvers. Most of the CFI applicants I've worked with have around 6 hours of dual in preparation for the checkride.
The real question is, how deep is your knowledge and how good are you at communicating and teaching that knowledge? Most of the time spent in preparation for the CFI checkride will be ground work. The more work you can do on your own to prepare yourself the less time you'll need to spend with a CFI doing the same.
Personally, I'd wait to talk to a CFI until you're ready to sit down and plow through the rating. From the time I started working with a CFI to the time of my checkride was about two weeks. I was working a full time job at the same time too.