I recommend either Gleim or American Flyers, both for different reasons which may help your situation.
Gleim: Last time I used them, two years ago, there was no minimum time required for each module of the course. In other words, you could open the section outline in one window and go immediately to the exam for that section in another window. That way you could refer to the outline for any questions you did not know the answer to. And because there was no timer or any minimum time required for each module, you could easily blow right through the entire course in a few hours if you really buckled down. Plus, it was cheaper than many of the other, at only $99. I do not know if their program still operates this way.
American Flyers: I switched to AF program this year simply to save money in the future as it is free to use for the rest of your life. It's more expensive than Gleim, but free next time and the next. However, there is a minimum time requirement for each section and the program will not let you take the exam for each section until you've met the time requirement. I was pressed for time and had my wife log on for me while I was on a trip and keep the timer going for me so that I could quickly take each test and move on. I would leave the timer running while I did other things. With my wife's help I was able to get it done in a couple of days.
I'm not recommending my study method as a good way to actually learn things or keep you proficient in instructing, but if, like me, you don't currently instruct and just want to keep your hard-earned CFI active, then it is certainly possible with minimal time out of your schedule.
Don't wait until the last few days of the month to start, or you will run out of time and your certificate will expire. After finishing any of the online courses, you will probably have to have the graduation certificate notarized and mail the information to the school. Or, if you live near a FSDO you can save several days. I recommend spending a few hours here and there early in the month and just get it done. Nobody says you have to set aside a full sixteen hours at one sitting.
