When does a tradition become a Tradition?

So the Catholic Church bases its beliefs on the truths revealed to it by God; that is, on Revelation from Jesus Christ and the Holy Spirit. This revelation comes in two parts: Scripture, which has been written down, and Tradition, which has been passed along by word of mouth.

In keeping with the Lord’s command, the Gospel was handed on in two ways:

• orally “by the apostles who handed on, by the spoken word of their preaching, by the example they gave, by the institutions they established, what they themselves had received—whether from the lips of Christ, from his way of life and his works, or whether they had learned it at the prompting of the Holy Spirit”;

• in writing “by those apostles and other men associated with the apostles who, under the inspiration of the same Holy Spirit, committed the message of salvation to writing.”

(Catechism of the Catholic Church, paragraph 76. The quotes are from the dogmatic constitution Dei Verbum of the Second Vatican Council, paragraph 7.)

Christ established his apostles, and their successors, to shepherd the Church; and he promised to be with it always, guiding and guarding it. The Church as a whole, therefore, and in particular the order of bishops in communion with the Pope, ultimately decide what is and what is not revealed truth. The God-given ability to do this rightly, under the guidance of the Holy Spirit, is called the Magisterium of the Church:

85. “The task of giving an authentic interpretation of the Word of God, whether in its written form or in the form of Tradition, has been entrusted to the living, teaching office of the Church alone. Its authority in this matter is exercised in the name of Jesus Christ.” This means that the task of interpretation has been entrusted to the bishops in communion with the successor of Peter, the Bishop of Rome.

86. All the faithful share in understanding and handing on revealed truth. They have received the anointing of the Holy Spirit, who instructs them and guides them into all truth.

87. “The whole body of the faithful... cannot err in matters of belief. This characteristic is shown in the supernatural appreciation of faith (sensus fidei) on the part of the whole people, when, ‘from the bishops to the last of the faithful,’ they manifest a universal consent in matters of faith and morals.“

(Catechism of the Catholic Church. The quotes are from Dei Verbum paragraph 10 section 2, and from the dogmatic constitution Lumen Gentium of Vatican II.)

Tradition in the sense I'm speaking of is not a liturgical or devotional practice handed down through the ages. Helpful as these may be for private and communal worship of God, they are not things passed along to the whole Church, as needful for our salvation. As you've asked for examples, we're talking about things like the various beliefs regarding the Mother of God, the understanding of the nature of the Trinity, the belief in Purgatory, and so on. Indeed, the decision regarding which books were to be included in the canon of the Bible is itself a matter of tradition, though not proposed as dogma until the 16th century.

Tradition can be developed in a number of ways. Sometimes, as with the Assumption or the Immaculate Conception, it begins with a very widespread belief and practice of the whole Church. Sometimes (as with the Trinity) it begins with a rather abstract theological discussion. Often, though not always, these are eventually set out formally as in the Creed, or propounded as a dogma:

The Church’s Magisterium exercises the authority it holds from Christ to the fullest extent when it defines dogmas, that is, when it proposes, in a form obliging the Christian people to an irrevocable adherence of faith, truths contained in divine Revelation or also when it proposes, in a definitive way, truths having a necessary connection with these.

(Catechism, paragraph 88)

So if the question is literally When does a tradition become Tradition? the answer is, it doesn't; either it is, all along (because it was given us by the Holy Spirit), or it never was. If the question is When do we know that a tradition is Tradition? the answer is more or less As soon as we see that the whole body of the Church must, has, and does accept it.