Whenever you're completely sure! A lot of people rush into saying it...Even if they don't mean it. It is annoying and how a lot of people get hurt. False "I love you"'s really can hurt. Be sure when you say it, please. It might save a lot of heartbreak. (If she doesn't say it back, she might not be ready, or sure. You never know!)
Just remember words said in the heat of passion can get you into trouble. Never in the first couple of months. You don't really know each other that early. It may just be a crush or you want to be in love so badly that you think you are. A good sign is to say it only when you think the other person is ready to hear it. you don't want to scare them off.