First, I've had a few drug tests in the last several months and nothing has come back positive. Besides, most times if it is random they select a few individuals then only select a few samples out of each group collected or they test for one specific drug.
Second, take PM from Ainterol, as I believe most have better success with this brand. I'm fairly certain i order mine from http://www.ainterolherbs.com/
Third, different amounts affect everyone differently. Start out with two capsules a day (1000 mg), then ramp up from there. Most start feeling the effects with 2000-2500, though again it varies depending on your body's reaction to it.
Finally, yes PM keeps "working" for some time after you stop taking it, but after you stop you make actually see somewhat of a decrease in size and shape. We are all experimenting here, so your results may vary.
Breast of luck on your journey!