Here's a study report on sodium hypochlorite (bleach) shelf life: http://www.forp.usp.br/restauradora/soda/sodaingl.html

It degrades somewhat regardless of UV eposure and high temperature though both of those things certainly factor in.

I wouldn't worry at all about bleach degrading on the shelves of the big box stores, especially Wal-Mart. Nobody but nobody even comes close to rivaling Wal-Mart's inventory control system. Nothing sits on the shelf there very long; especially consumables like bleach.