I have seeds that had been in my fridge for 4 months. I planted a few recently and they sprouted. As to how long beyond this time frame they would still be viable, I couldn't say. Maybe somebody else could pipe in and answer you.
I could not give a exact length of time, but if stored properly after any extended period of time. You should expect a lower rate of viability.
This how I store mine.
"Many times a day I realize how much my own outer and inner life is built upon the labors of my fellow men, both living and dead, and how earnestly I must exert myself in order to give in return as much as I received"