There is no "best time" for a game announcement. Some people want to have informations far in advance, others prefer announcements close to release.
Port announcements should be as early as possible from a customers standpoint to prevent settling for the "wrong version" of a game. Publishers love to hide these informations as long as possible, hoping for "double dippers" and due to NDAs with Sony/Microsoft/Nintendo.
Late announcements without leaks/rumours are hard to realize nowadays because almost every developer has its fanbase. "Developer X just released game Y... what are they working on now?" - "Let's collect every snippet of information we can find on blogs / Facebook / Twitter / Xing... accounts of every person (programmers, designers, artists, voice actors...) working on their games!"
