The only specific time requirements apply to the MEI. Those are 15 hours multi PIC, and 5 hours in make and model to teach. You can do the CFI really fast. But to feel ready, it will take a ton of time.
Personally, I don't think I could have done the studying at the same time. To be a good instructor, you have to re-teach yourself a lot of stuff that's been forgotten, or simply become automatic. I passed my initial a couple weeks ago and I elected to just focus on getting some time with students before starting to work on the II and the MEI. I'll probably do most of the work in a twin because I've only got 2 hours multi and in the long run it will save me some cash.