exactly 3 minutes? thats hard to say. i could see around 3 minutes considering that razor burn is caused by irritation which causes inflamation which your body should try to heal. this would take time, just like it takes time for a cut to scrab over. i don't think that if you wait longer that putting anything on will do nothing also, but i guess it would depend what you put on it also. plus the healthier you are the faster and more efficient your immune system will kick in to heal your face.

these are just my thoughts on it though.