8wks should be fine. I have, as well as more people than I can count have used it longer with no long-term negative effects. 6-8wks is however a good safe rule of thumb.

15mcg isn't going to do much. 25mcg/ed is considered the lowest baseline dose to treat hypothyroidism.

The reason you ramp it up is so you can promote stronger fat burning as you need it. You may or may not have to ramp up all that high, then again you might...you kind of have to play it by ear. The ramping down and especially that final wk at 25mcg/ed is the most important factor. It's a bad idea to just come off T3 cold turkey. It's much easier on the thyroid to re-accustom to lower levels of the thyroid hormone before dropping it altogether.

The only reports of anyone suffering from hypothyroidism due to the use of Cytomel all surround extreme overexposure...we're talking about extremely long-term use.