How Long Does Niacin Take To Clear The System?

i have taken diazapam or valium, last dose was 2 weeks ago. i work out 6 days a week, and sit in a steam room for 10 minutes after every workout, all i drink is water. i have been taking flush niacin about 1000mg every 4 hours with lots of fluid. how long will it take to clear my system? i have a very important job interview and drug test next week.