You shouldn't ice for longer than 20 minutes due to the risk of freezing your skin (even when using a towel as protection) which can result in skin and nerve damage. I must say I didn't know that at my first surgery so I definetly iced way longer. I think my knee is very used to the icing. I am icing at least twice a day (after lunch and before bedtime). I sleep with my icemachine that activates every 80 to 100 minutes for 10 minutes, sleepsaver!