The table at the bottom of this link will give you an idea of the stability of chlorine at various concentrations and temperatures. Roughly speaking, chlorine degrades twice as fast as the square of the concentration and with every 10F increase in temperature. For 6% bleach, the half-life at 90F ambient temperature is around 330 days, but that's for well-made bleach without contaminants.

I suspect that direct exposure to sunlight had the liquid in the container go to 110F if not more and that would have been a half-life of 80 days or so, but a drop to 5% would occur in about 20 days. Even with an opaque container, it still probably got quite warm in direct sunlight, especially if it wasn't all pure white.

I keep my 12.5% chlorinating liquid in a shed, but the temperature usually only varies from 60F - 80F and doesn't get above 90F very often. I've only noticed a small degradation from 12.5% to around 11.5% after about a month during the hottest months.

Richard