Saint John the Baptist was recognized as a saint from the very early days of Christianity. He was not formally "canonized" in the way that saints are today, as the formal process of canonization developed much later. However, he was venerated as a saint from the beginning due to his role as the forerunner of Jesus Christ and his martyrdom.
Therefore, you could say he was recognized as a saint since the very early Church, around the 1st century AD.