If someone says "I will love you a long time", that is not a compliment! That means until he decides a long time is over now. That is no promise! If a man promises to love you "forever", well, maybe that is saying something. Maybe he is wanting to spend his life with you.
No, a boyfriend and girlfriend would not say such a dumb thing. Not even someone who wants sex. That is no promise at all! Who is to decide what a long time is? No one knows such a vague statement.