There's two parts to this question. One is how long will deca keep acting as an active anabolic in your system, and the second question is how long until deca is no longer detectable in a drug test. The answer to the first question depends on the dose you are using, but deca is very long-acting so it should be actively anabolic for at least 2 and as many as 6 weeks after your last shot. The answer to the second question is deca can be detected in a drug test for a long, long time - as much as 18 months later.