It's been over a year; it was first announced at the 2014 E3 event, and almost certainly was being worked on before then. It's actually very similar to Star Fox Command, which was announced 2005 and released 2006 even quicker; it was an August release, actually. Going the other direction, you have Star Fox Adventures, which had a long, drawn-out development, even by the standards of Rare's often long, drawn-out development cycle.
It's been over a year; it was first announced at the 2014 E3 event, and almost certainly was being worked on before then. It's actually very similar to Star Fox Command, which was announced 2005 and released 2006 even quicker; it was an August release, actually. Going the other direction, you have Star Fox Adventures, which had a long, drawn-out development, even by the standards of Rare's often long, drawn-out development cycle.