Ice therapy should be used during the first 72 hours following an injury. If you have poor circulation, diabetes or other medical conditions that would make using ice dangerous do not use ice without speaking to your doctor.
After an injury, ice will be used to decrease the inflammation and swelling to the area. Do not put ice directly on the skin, but you should have a towel between the ice and the area you are applying the ice to. You can leave the ice on for 15 minutes per hour for the first few hours then may apply ice every 2-3 hours after that.
Rest and elevation along with compression may also help.