The title of this article is causing much confusion on the net; shouldn't you all at ANN edit it? There hasn't BEEN a second season of SAO yet, not even one announced as far as I know. There's been a second half to the FIRST season, which had 25 episodes, but no actual second season has been confirmed yet.
And for those of you saying there are 4 seasons to a every year, while that is true, not every show follows the 13-week season structure. The amount of episodes for any given "season" is what the production company says it is - for some series that's only 10 or 11 eps in a season, for others its 25, 26, 52, or 39 - the most popular amount of episodes for a series to have in a season is certainly 12 or 13, but it's by no means a rule or the standard.