This question requires a two-pronged answer.
When God created man, at the beginning of creation, man recognized God and worshipped Him in the way God ordained. In this sense, the worship of the Christian God and reliance on His grace (which is the essence of Christianity) has been around since the time of the very first man, Adam.
However, a proper understanding of God's plan for mankind has slowly been revealed by God through His prophets over the course of several millenia, culminating in the life, death, and resurrection of Jesus Christ in the first century AD, approximately 2,000 years ago.
Christ's closest companions, called his Apostles, and a few other followers, wrote down what they witnessed and what God had divinely revealed to them in what we call the New Testament, a set of books in the Christian Bible.
As of the completion of these writings, no new universally applicable special revelation has come to man from God, and therefore, in a different sense, we can say that Christian thought, as it is today, has been around for approximately two thousand years.