"Monitor and control the process against the plan for performing the process and take appropriate corrective action."The intent of GP 2.8 is not just to monitor and control process, as indicated by the title, but to directly monitor and control the process against the plan for the process (GP 2.2) as you PERFORM the process. Many organizations think that by checking some process measures on a monthly or quarterly basis, after the fact, is meeting the intent of GP 2.8. The frequency of monitoring and controlling the process depends on how often you perform the process. Some process occur weekly and others may be only once or twice during the project's lifecycle. So, once again, the monitoring and controlling of the process must occur as you execute the process. Keeping in mind that reporting the results of the monitoring and controlling effort can happen on a monthly or quarterly basis.
The other misconception about GP 2.8 is that you can read the practice and interpret the words to mean that you can monitor and control the process without using data. That is true if you just read the words of the practice title. However, the practice states that the monitoring and controlling of the process is based on the plan for performing the process. Once you are using a "baseline" for comparison, then the implication is that you have some means for measuring and determining if there is a need for taking appropriate corrective action. In fact, the GP 2.8 discussion in the CMMI refers to the Measurement and Analysis (MA) Process Area for more information about measurements. So the intent of GP 2.8 is to use MA to define one or more appropriate process measures and indicators that will be used to measure the process' performance against its plan, as the process is being executed. The GP 2.8 sub-practices communicate the intent of this Generic Practice. Taking corrective action should not be because an arbitrary threshold was exceeded, but instead the criteria for taking corrective action should be when the process requirements and/or objectives are not being met, process issues have been identified, or when process progress deviates significantly from the plan for the process.