At my work we buy 12.5% in 330 gallon totes, and our summers are 100+ degrees. This is just anecdotal, but in my experience if it gets stored outside in the sun it starts deteriorating pretty fast, but if we keep it under a shaded area (not indoors) it lasts a lot longer.
I've also purchased 10% from local Lowe's and Home Depots, both of which store it outside in the covered garden area, and have never had any problems with potency. Again this is in an area with really hot summers. Not sure about where you are, but here the big box stores package it in cardboard boxes that don't allow sunlight through, so that really shouldn't be an issue.