I've been taking advicor for five years and now my doctor is switching me to zocor (simvastatin). How long does the niacin stay in your body. Flushing?

Good question. I researched your question: the half life of extended release Niacin in Advicor (lovastatin and niacin) is very variable, & depends on gender, dose, & individual characteristics. It peaks 5 hours after a single dose. Following years of treatment, it's probably safe to assume that it will take days (3-5) to wear off completely. Possibly longer depending on dose.
Niaspan (niacin) The Niacin should be out of your body within 24 hours.
Less than a week. I usually taper meds down and then switch to the newer medication just to make sure there is not an interaction with muscle aches since the two medications together can cause this problem.