It was probably less than a year, and you forgot. But yeah, those things last a really long time in sleep mode. The DS can last for ages in sleep mode, but I doubt it would last anywhere near a year. Wikipedia claims that it can last 7-12 days. But then again, it is wiki. (So it may be longer, or shorter)
EDIT: Dread is a BAMF for all of you who didnt know.