Ive not used "fresh" seed in ages. I buy on close outs at the end of the season, mostly, so Im almost always planting last years seeds (or older).
SO far, about 3 years is the practical limit for most store bought seeds, if they are kept free of damp and excess temps during storage.
Remember, most store seeds are hybrids of some sort and may not have the vigor of long established lines. Viability is likely to fall of quick with them, so Id consider replacing 3 year old seeds as a rule of thumb.