Reliability growth plans are typically described by using reliability growth planning models that generate growth planning curves. The PM2 model, also known as the planning model based on projection methodology, is a widely used growth planning model. The PM2 model can always meet the assigned reliability target during any reliability growth testing duration, regardless of changes in the management strategy parameter and/or the fixed effectiveness factor parameter. In fact, these two parameters determine whether reliability growth testing is successful or not and how long it takes. That is, the PM2 model currently in place needs to be modified accordingly. Firstly, a sensitivity analysis is carried out on the three parameters of the PM2 model, followed by a discussion on the correlation between the product of two process management parameters and the duration of testing. Then, a relationship equation for the testing duration is constructed using these two process management parameters. Finally, an extended model for planning reliability growth has been proposed. In addition, the proposed planning model is utilized to examine an illustrated example and a real case. The results have demonstrated that the model is reasonable and reliable, as the subjective input, testing duration, was removed from the reliability growth planning model.