It's ten years for cars. I would imagine (if it's built to last?) 10 years could apply to furniture as well. Particle board with wood laminent will never last that long. Ethan Allen furniture or even hand made furniture with fine craftsmanship will survive to be an antique.
I think that furniture can be considered antique if they are between 50 and 100 years old. However, it is common for objects to be considered antique if they are at least 100 years old. For more information, go to http://en.wikipedia.org/wiki/Antique.