I know all the technical answers of charging cycles, etc... but... I still have/use my trusty old Anker 2nd Gen Astro3 12800mAh which I purchased through Amazon December 2013. It still works and holds enough charge to re-charge my phone about 3 times.
My wife also has an Anker 2nd Gen Astro3 12800mAh purchased July 2014. She uses her's for her iPad and iPhone... still holds enough charge for her to not ask me for a new one yet. And for the last year+ she has been flying around the country for work, taking/using her Anker in the airports/airplanes, on Cub Scout camping trips (since I am unable to camp), etc.
If you go by the "technical info" these batteries should be degrading, but they still work great.
That being said, I did get a few other Anker power packs since then. Just different capacities and features since I was traveling more and upgraded phones... I have some like the PowerCore 20100 for capacity (longer trips, or used in Disney Parks when on vacation)... PowerCore+ 10050 for QC support, and a PowerCore Mini+ 3350 for discrete on the go charging...
But my wife still uses her's, and other than our last trip to Disney World, just to play it safe, hasn't borrowed any of mine. On that trip, my son used my Astro3 on the plane, and our friend (who met us down there) used my Astro3 in the parks.