I've been looking through my seed viability records. I've only kept records 2 years and we're not very far into this year yet, but this is what I have for how long my seeds last.
Arugula 5+ years (3 years)
Beets 3-4 years (4 years)
Cabbage 6 years (4 years)
Endive 3-4 years (5 years)
Leeks 3+ years (1 year)
Onions 2 years (1 year)
I put the viability that High Mowing Seeds gets in (). I guess my numbers are pretty similar. They keep their seed under ideal, very consistent conditions and they have lots of data. I suspect they define less than 50% germination as not viable. I don't have much data, like only one 6-year-old cabbage package that sprouted fine (but I don't know the %) and 7- and 8-year-old packages that failed.
I'm looking forward to see what I get for my tomatoes. It always seems like they last a very long time. High Mowing Seed's chart says 5 years.