It is always best to CLOSELY follow the instructions and recommendations of the treating physician. The time to begin the safe use of sunscreens depends on the type and intensity of the CO2 laser treatment, the particular sunscreen you are going to use, and how your skin is healing. In general 4-5 days for lighter treatments, and up to 7-10 days for more intense treatments. Much will depend on the healing time of you skin, which should be determined by your treating physician. But the fact that you are thinking about protecting your skin from the sun (which also includes sun avoidance) will be a significant benefit for your overall result.I hope this helps.