While all this is great and all, there is no right or wrong time to say "I love you".
I usually wait till I love a girl, before I say anything about love. They will usually say it before I do though.
But I don't just 'fall' in love. I actually love a girl before I say it. I might love her like a friend, and then I will say it, or I might not say it, because it might make things too tense, and put sexual tension on the relationship.
If you love a girl as more, and you know she does, it should be ok to tell her so, but not everyday, or all the time. Of course, if you love a girl, but are stupid, just keep your mouth shut. If she is stupid, and you are stupid, you are going to regret saying I love you.
Of course your 'I love you's can be mutual, and then you don't have to say anything, and can assume that you both love each other.
But most people think they are in love, and are in fact, not in love. It's hard to tell. Most people aren't the famous INTP personality type that I am. You really have to meditate, and consider if you are in love, but most people are too stupid to tell if they are in fact in love.