If you ask a grumpy aunt or a church calendar, the answer is Twelfth Night. That’s January 5th or 6th, depending on whether you count Christmas Day as Day One. This tradition dates back to the 16th century, when people believed that leaving decorations up past this date invited bad luck.
Specifically, the evil spirits might move in. Because apparently, ghosts are highly allergic to un-taken-down garlands. I’m not saying your blinking Santa is a demon magnet, but… maybe don’t test it.
For the record, the Church of England is very clear: down by January 5th. Miss that window, and you’re technically living in liturgical sin. You’ve been warned.