Chlorine concentration does degrade over time. It ap[pears to be related to temperature. Here's a link: http://www.odysseymanufacturing.com/about_product.htm. At the bottom of that page is a link to shelf life of chlorine at different concentrations.

I buy liquid chlorine which is said to be 14% when manufactured. I've tested it a couple of times and have come up with something between 10 and 12%. I use 11% when I calculate how much to add to my pool. When I have chlorine left over at the end of the season I don't actually care what strength it is at startup. I just pour in what's left because I know the pool will be at 0 FC.