Predicting how long it will take for marijuana to completely leave your system is not an exact science. Marijuana can take anywhere from 2 to 6 weeks to leave your system, so the safe thing to do is to assume it will take at least one month.
So long as you have a prescription for both from your doctor, you can take these drugs together. Remember that Concerta is a narcotic stimulant, and can be habit forming. Take it exactly as the doctor says and do not try to break open the pill. Do not take extra Concerta.