According to Clifford White's Handbook of Chlorination, the half-life of 10% chlorine solution at 77 degrees (F) is 220 days which is not too bad. It is also my experience that if there is a decent turnover of chlorine bottles and they are kept out of the sun, then I tend to get properly concentrated chlorine from my local pool store. I get the expected increase in chlorine levels by adding an amount of chlorine based on the 10% level (the 10% is by weight of NaOCl while the 12% available chlorine is by weight of the HOCl you get by putting the chlorine in water).