I'm still using 2016 Willamette hops that were frozen in their original packaging since I received them in late 2017. The HSI for Willamette is ~38%, so their AA% is now under 3% but they still work well for flavor and aroma additions. These came from Yakima Valley Hops, so I'm confident that they were handled and stored under optimum conditions, which is an important factor.
Once I open the original packaging for use, the remainder is stored in a vacuum sealed Mason jar and generally used within 6 months.