^Since The Brave and the Bold
ended in late 2011, at least. As the article says, they typically have five episodes in production at any one time, overlapping.
What worries me is that with such lead time, the show would have to get a renewal order very early in the airing of one season in order to get the next season on the air in a timely fashion. Certainly BtB has made an impressive debut, so I don't think we have to worry much, but I hope the renewal comes soon enough that we don't have to wait until something like late 2014 for the second season.