I used some ID-11 yesterday that was mixed 11th April 2004 ("Happy birthday to you, ... Happy Birthday dear ID-11, Happy ..." etc, etc). The bottle has been 3/4 full for about 3 months and kept in a dark cupboard. Very, very, very, slight colour and worked fine. The stock was made up with London mains water (i.e. more calcium than most people's shin bones). No help really, but does indicate that under normal conditions ID-11 stock, will last quite a while.
As this is the last of that batch and it's time to make some more (and there is now less than 100ml in the bottle) I will, in the interest of science, take a few ml out and let it oxidize to treacle and then add it back to the bottle and see if it effects the good stuff. I expect no effect, but we shall see.
Cheers, Bob.