Oh man...I kinda feel bad admitting this. I currently I have 106 bottles open.
I did a semi-scientific test over the past couple of years, and found that as long as there is at least 1/3 of the bottle full it doesn't seem to affect the flavors much at all. I had several bottles that I had bought "doubles" of 3-5 years before, and then did a head-head test between unopened and stuff that had been open a long time. In general I found that the heartier malts were really difficult to distinguish, and the harsher malts tended to age out to smoothness. I had one bottle with only two fingers in it, and that one had turned really stale. Of course I store them in a light-sealed armoire usually in the original tins, so that probably helps.