Don't talk in weeks. A few days beforehand is about right. One week before is too early, so anything after the 18th of December. What many people do is put them up a few weeks before it and then take them down within a few days of Christmas Day, when in reality Christmas has only just begun. It lasts 12 days, though people seem to forget that and take them down at about day 2 or 3, having had them up for weeks. So they put them up long before Christmas begins and take them down long before Christmas ends. So putting them up even on Christmas Eve is not too late. Keep them up throughout Christmas, seeing as they are Christmas decorations, and then take them down on the 7th of January, when Christmas has just ended. That is the sensible way to do it.
four weeks
three weeks
two weeks
six weeks
Many people in the US decorate the day after Thanksgiving.
Germany, where many of our Christmas traditions originated.
it depends on what day it is. If you want you can check on your calendar to see how many weeks it is until Christmas
Weirdly, many families in France decorate their house with mini Santa's climbing up a string to their roof/balcony, villages have similar Christmas lights to england.
6 weeks.
18 weeks
many people make shops into Christmas Only shops, people decorate, and air Christmas music on the radio or movies on the TV
Many commercial companies opt to decorate their stores/business locations with decorate Christmas ornaments with lights or if trees are readily available it is very appealing to wrap the trees with lights.
Many Japanese people decorate their homes with evergreens and Christmas trees. They decorate with lights, Santa figures, and other seasonal decorations.
About 14 weeks
9 weeks and 2 days.
Many choose to decorate during the Thanksgiving weekend. Too early depends upon the person, I have decor I have not taken down in two years as they are more troublesome to put away than to leave out. Others allow Thanksgiving to take place and decorate anywhere between holidays.