Slim to none. Never had any seed that old but every year that goes by diminishes the viability. Some plant types are worse than others but that is the reason you don't buy in bulk and keep planting your garden from the same stock each year.
Still, I could not resist taking a couple groups of 100 seeds each and sprouting them on a warm moist towel to count the percentage, if any, that germinate. If you do that I would love to hear the results.