If you store them properly, a long time. I've heard people successfully germinate and grow 15 year old seeds.
Yeah well I guess proper storage would be the best policy. I've been out of the game for a few years and upon cleaning out my closet I found some seeds. Most are @4-5 years old, mostly Nirvana seeds. I germinated 12-15 of them, some sprouted tap roots others didn't. Strange thing is that all but 1 of the seeds that actually sprouted a tap root died. They just stopped! I can only assume that its the age of the seeds and the conditions they've been kept in. I haven't changed any of my "methods" since my last grows so I cant blame that. So anyway...short answer is care for your seeds....I too have heard of 15-20yr old seeds doing well.