I say it to my wife and youngest daughter every day, because I see them every day, and to my older daughters when i see them, which is usually once a week at least.
I first said it to the love of my life when I was 21.
Well if i can be a bit pedantic, ddaisy, and take your title at face value- "i think i love you", then it's the sort of thing i would say to a shop assistant who goes out of their way to help me find something! x