What is the ideal amount of time that instruments should stay in an ultrasonic unit with settings of 6, 12, 20, 30 and 60 minutes?

Since ultrasonically cleaning instruments does not sterilize them, there is not a mandatory time limit for the cleaning process. The purpose of ultrasonic cleaners is primarily to debride the instrument surfaces before sterilization. Therefore, the ultrasonic process is directly related to the amount of debris left on instruments. If you are finding that the setting you are using does not completely debride your instruments, you can either increase the time or use ultrasonic pre-soak solutions.