I accidentally said it one day when we were saying goodbye, it was only a few months into the relationship and he just looked at me. I did strongly care about him, and a few days later he told me he really cared about me to, and said it back. I think it was a bit longer before we both knew we were really in love though. But I think that's how high school relationships go. haha.
He said it very quickly, and swears he meant it. It took me quite a bit longer to feel it and then say it. It takes me a while to fall in love. I'm not sure how long it took me, but it was in the realm of months.