I usually start some time between 1/1 & 1/6 (Epiphany).
We have TONS of Christmas decor, &, if I don’t start taking everything down until 1/6, then it’s usually around 1/8 or so before everything‘s completely put away.
Our DD’s bday is 1/5... some years, we’ve had all the Christmas put away before her bday, &, some years (especially in the last 10 years or so), we leave it up for her bday since her bday is technically the 12th day of Christmas.
Growing up, my mom always put away the Christmas decor on 1/1, so, when DH & I were first married, I tended to carry on that tradition.
However, in the last 10 years or so, we’re gradually changed to more of a Epiphany tradition. I learned that the 12 days of Christmas begins on 12/25 & ends on 1/5 & thought it was kinda neat that our DD was born on the 12th day of Christmas.