Nandrolone has a half life of six days. As for the test it depends on what type you used. testosterone cypionate has an eight day half life. Testosterone enanthate the best I could locate was six to 10.5 day half life. Propionate has a half life of 4.5 days. There are other esters but just so you can see it depends on what you have taken. For the purposes of effect, typically after three half-lives for many drugs their effects are dramatically reduced if not no longer measurably effective (excluding drugs that depend on metabolite action). For clinical purposes five half-lives is considered a drug to fully expressed or excreted.
So to answer your question, after 18 days of nandrolone (deca) the effects will likely be negligible. As for the test, that would depend on the ester. However I be looking for a solid PCT to start after the third week. Please see the thread below.