It's debatable, but I'd say years If stored properly. I just used Galena hops that were 5 years old (vacuum sealed and frozen) in the best IPA I've ever made. Brulosophy did an experiment on hop age that you may find interesting...
Adventures in Homebrewing has 1lb bags of Cascade hop pellets for $6.99+ shipping. These are the 2016 crop. Shipping was a flat $11 for me, no matter how many bags I ordered.
https://www.homebrewing.org/Cascade-Hop-Pellets-1-lb_p_690.html
Finally brewed this one today. I substituted The hopshot, nugget and Chinook additions with Galena pellets because I have a ton on hand. It's bubbling away like crazy and smells amazing. I'll be sure to post impressions in a month or so.
I've been to 14 of these cities and Stockholm is hands down the most expensive I've seen. It's way more expensive than NYC and Paris. $8+ was the norm. Not only that, but beers purchased in store are also very low ABV and relatively expensive (all sold individually). I've attached a pic I took...