The wordplay here between "belong" and "be long" was explained by Paul McCartney: "I was doing literature at school, so I was interested in plays on words and onomatopoeia. John didn't do literature but he was quite well read, so he was interested in that kind of thing. Like the double meaning of 'please' in a line like 'Please, lend a little ear to my pleas' that we used in Please Please Me. We'd spot the double meaning. I think everyone did, by the way, it was not just the genius of us! In 'It won't be long till I belong to you' it was that same trip. We both liked to try and get a bit of double meaning in, so that was the high spot of writing that particular song. John mainly sang it so I expect that it was his original idea but we both sat down and wrote it together."
Another Beatles song to use "be long" repeatedly is "Blue Jay Way