well, you CAN use nothing, or go with the thermo on your off days. it's really up to you. i used a thermo in my off days b/c 1) i wanted (and needed) the extra energy boost and 2) i also wanted the other fat burning/metabolism/appetite suppression benefits on those days also. it's just to accelerate/enhance the fatloss and make the process "feel" a little more pleasant if you're dragging tail badly. if you are more inclined to overall well being, balance and health, i would just use green tea on your off days.
as far as how long, it varies. i've found 2days/2days can allow it to run longer. some people use it until they reach their goal (weeks, months), some until they feel it loses effectiveness or have reached a point of diminished returns. just remember, it is only a tool. i'd only use it sparingly and with a specific goal in mind (contest, photo shoot, vacation, wedding, etc.), then allow the body to get back to "normal" and learn to accept what you can create with diet, cardio and training.
as much as we'd like to be these walking works of perfection, it's sometimes good to step back and ask ourselves, am i trying to live up to someone else's standards of beauty/attractiveness? am i sacrificing my health and well-being for this? will i know when to back off or stop?