How long do the drugs for chemotherapy and rituxan (rituximab) stay in the system after you're finished?

Up to 3 months. Chemotherapy leaves the body very quickly, usually within 24 hours. Rituxan (rituximab) is a monoclonal antibody, which is a protein. It's half-life is one-week, meaning it takes 7 days for half the amount to be gone from your system, and another 7 days for half of the remaining amount to be gone, etc. Rituxan can still be measured in the blood up to 3 months after a dose!
Depends on the drug. Rituxan (rituximab) for RA can have an effect that can last for even over a year . For cancer the dose the other chemotherapy drug duration will depend on the drug itself.