When is the New Year No Longer New?

Happy New Year!
When is it no longer the "new year"? When does it just become, "The year?"
If on January 8th, someone came up to me and said, "HAPPY NEW YEARRRR!", I would be compelled to say, "Dude. Shake off your hangover. That was like, a week ago."
But why?
The year, if you think about it, is still relatively new. When is it no longer new? Is it in February? Is it in March? Is it January 10th?