You can't get your initial CFI until you have an instrument rating, so I'll assume that you need to get both your CFI initial and CFII.
Depending on your ability and how well you retain information, at 4-6 hours per week I'd guess about a month. This is based on the initial CFI being mostly ground school stuff...(the flying you've already done, you just have to get used to the right seat, and that doesn't take long). Get through the dirty half dozen and your ground and take the ini CFI at 3 weeks, then add on your CFII in the 4th week.
You'll have to have your commercial certificate first, but there's no reason you can't do your CFII as your initial flight instructor rating. It will probably take a bit longer than it would if it were an add-on because you'll have to become familiar with the Fundamentals of Instruction as well as the IFR material. The CFII written is from the exact same question pool as the IFR, so if you didn't already take it, it shouldn't be too difficult for you to refresh you memory on the stuff.
From where it looks like you're at now, you'll need to do this:
Take the Commercial written
Do the Commercial training and practical test
Take the Fundamentals of Instruction (FOI) written
Take the CFII written
Do the flight and ground training for the CFII
Take the CFII practical test
At 4-6 hours per week you're probably looking at 3-4 months for your commercial cert. since you'll need 250 TT before you can take the practical (unless you're part 141, then the time restriction is less). After that I'd give it another 3-4 months since it is your initial flight instructor rating and it takes a little extra ground work.
I only flew 1-2 times per week and my initial CFI (I did it the traditional way and went CFI then CFII) took me about 9 months to complete. Of course I was also flying for fun in between so it stretched it out some. I added on my CFII in about 4 months, squeezing in about 6 flight lessons and 6 ground lessons in between teaching students and working a 40 hr/wk day job.
You seem to have more time and resources to dedicate to it, so there's no reason you couldn't complete them in about 1/2 the time I did.
I would advise against doing the cfii ride as the initial one. The cfi checkride is hard enough already and if you attempt to start as a cfii or mei an examiner might view that as arrogant and cocky and give you an impossible checkride.
The cfii shouldn't take longer than a month. Personally, I'm a big fan of accelerated courses for add on ratings if you can arrange that with a cfi or business somewhere. For example, a 2 to four day program for the cfii could finish you up quickly. Good luck and get those writtens out of the way.
I did my MEI first at Vegas and already had 100hrs of multi so it wasn't that bad. But, the check ride was a handful, and I did every maneuver that seems possible. The CFII and the CFI were cake after the initial. The check ride is only as hard as you want to make it "mentally" and as hard as the examiner wants to makes it. So definitely track down an examiner that is easy going and not trying to prove something. My ground was 3hr while the flight was about 1.9. I know some that have ground time of 6-8hrs. Even if I knew everything, I still wouldn’t want to sit there for 6-8hrs.