It depends on your body and its current health. Cell duplication can occur as short as 60 minutes, and repair process as quickly as a few seconds. This did why we encourage to make sure to always have every bioavailabilities ready for the body to use.
So, it depends widely on your body health. If the body has the capacity to enforce cell repairs and duplications, all you need is that 40 to 60 minutes. This is why we remind the brothers to wear the cock rings and extenders/traction devices for the 60 minutes post routine. How to predict your body is growing? 3 to 6 months with measured growth, including the hours of wearing. That allows a calculating factor of growth using geometric and predictive growth models. Not the answer you're looking for, but close enough to tell you, "stay healthy and be diligent."