Studies show that putting ice on the skin over the knee starts to cool the joint in the first five minutes. The cold decreases the blood flow and slows down the metabolism of the bone and muscle.
The maximum benefit from an ice pack has been measured at 20 minutes. After that the body will send more blood to the area to try and warm it up. It's usually advised to stop cold treatments after 20 minutes to avoid nerve damage and to prevent the increase in blood flow to the area.
Other studies using cold for up to one hour don't show any damage from the long cooling period. With the development of new temperature systems in the last few years, we may be able to find out more about the effects of long-term cooling.