Thoughts:
Some eastern european traditional foods contain potatoes and tomatoes and yet they are considered traditional though they have been used for cooking only for about two centuries.
Some instruments like violins were scarcely available in rural regions until the late 18th century yet traditional folk music is based on violins.
Some traditional clothing have strong purple motifs like flowers or birds although purple dye was not available to common people only after the industrial revolution.
*All these examples are based on folk traditions from eastern europe (Bulgaria, Hungary, Romania, Moldavia)
The benchmark publication on this, a collection of essays, is Eric Hobsbawm and Terrence Ranger, The Invention of Tradition (Cambridge: Cambridge University Press, 1983). The work in this book attempts to address your question. In general, the answer to your question is "not much time at all." It is not possible to quantify an exact number of years - or even a general range of years. Tradition - for all the term seems to imply - is not very traditional and the traditions can be extremely fluid with the adoption of new practices becoming traditional very quickly - or sometimes over longer periods of time.
At the same time, some traditions seem embedded and are not easily changed. Basic assumptions - things such as the gender of the sun and the moon, whether the dead return in hazy spiritual form or in corporeal form, whether fairy-like supernatural beings appear in families or singular (whether in ones, twos, or threes), things happening in a specific number - seem to be tenacious and slow to change.