How often should one water Christmas trees?

About 2-3 times a week. Your best bet is to check the water level in the holder your tree is sitting in. If your fingers touch water, you're ok. If you feel no water, add about 2 quarts at a time. If that's inaccessable, fill up the holder the first day you have the tree, and every day afterwards add just a cup or two. That should hold you over.

I also find that using a spray bottle to keep the leaves wet or at least damp does some good for the tree.