Actually in lots of places the trees are likely to last longer if you get them up earlier and get them into water. They're all cut in bulk and transported to where they need to be. Some suppliers will keep them in water but plenty don't.
Good point about getting it in water early. My main concern is that the only place it can really go in the living room is next to the radiator. Masses of water would help though - I don't think we've ever given as much as you describe, Silly.
It absolutely has to make it to 6th Jan. Our options are basically to get one and put it up in the next few days or wait until the 24th - when the best trees will be long gone!
Buy from a tree farm. Ones from garden centres/ supermarkets etc are often cut down in August or September or earlier, stored then delivered to stores in December. Home grown ones are much more recently harvested, so last longer. We get ours late November and they hardly seem to drop. Last years stayed green til I got around to getting rid of it in late February!