we have got a pine tree happily growing in a pot outside for a couple of
years now. Its now about 1.4 metres high and I think this picture is the
same type of tree.

http://7art-screensavers.com/wallpap...-pine-tree.jpg

We would like to bring it in over christmas and decorate it. It would be
near a southfacing window in a living room. We have central heating so the
temperature would for the most part be about 21 C.

What would be the maximum length of time we could keep it in these
conditions before it got seriously upset do you think?