It's impossible to give an exact answer, there's no exact timeframe to say when one will get clean, just too many variables to factor in. You're a light smoker though, twice a week, and you haven't smoked in the last 2-3 weeks. You'll be fine for a drug test, just don't overdo it though with the water, the last thing you want is to be flagged because your urine _is_ diluted.