A demo will probably only be available close to the launch date, 2-4 weeks in advance at the most. First, because the demo is just the shipping version of the game with a lot of the options stripped out (in Civ's case only 1 or 2 civs and a limited number of turns), and second, because the effort that is invested in a demo should translate into a momentum enthusiasm for the game that should result in sales. And even though you can pre-order the game well in advance, a lot of people wait until they can actually buy it, so you would waste part of the effort if you release a demo too far in advance of the release date.