When someone gets an injection of T every week or every two weeks would the T stay in the body for that period of time making changes to the body or does the T only stay in the body and make changes during the first day of the injection?

It lasts up until the next injection. If you're on injections the levels are never really stable. It starts lower, inclines, and slowly decreases until it's time for the next injection. If you use gels or patches the dose in your blood is far more level. Most guys do prefer injections because of cost but also because the "high point" it reaches for a few days after the injection is what causes masculinization sooner than other methods.

Oh okay. It was just a random question that popped up in my head.