How long does 20mg of oxycontin stay detectable for a court administered drug test?

It's all Pharmacokinetics. The dose, duration, frequency and your weight have nothing to do with how long the metabolites of oxycodone are detectable. It's all in the half life, elimination half life to be more specific. It takes 7 elimination half lives for the amount of drug in the urine to fall below detection levels and >9 elimination half lives for the remaining drug to be deemed insignificant.