Only because I'll do just about anything to keep from doing my stupid homework:
The values start to drop to about nil after day 21 FYI. Keep in mind, milligrams of test do not equate to a specific number in your blood. I would just stick with 2-2.5 weeks and call it good before PCT starts. Sure, you'll have less exogenous test in your system if you go all the way to 3 or more weeks, but I don't think it's really necessary.
My .02c