I took .5 clonazpem. how long will it be in my system?

Long lasting: Half-life is 18-50 hours depending on your metabolism so it may be present for about 5-15 days depending on how efficiently your body breaks it down.

Half-life = 30-40hrs: Just curious: did you take 1/2 tab (if so, what dose?) or 0.5mg? Regardless, drugs stay in your system about 5 half-lives. After 1st half-life, there's 50% of original left behind. After 2nd half-life, there's just 25% of original. After 3rd half-life, there's just 12.5% of original. And so on. In case of Clonazepam, let's be conservative and use 2d as half-life. So you should be clean 10d later.