Good points - most of my experience with rechargables is with NiCds from at least a couple years ago, and indeed the Li ion batteries are much better with respect to charge retention. The NiCds and NiMHs seem to be a bit better too, though still inferior to Li ions.

I do remember one experience a while back where my spare battery (Li ion, fairly new, initially fully charged, stored in a small plastic bag) turned out to be dead after about a month of idle storage. I don't know how it ended up like this but it did. I suppose it's a cautionary tale.

I'm not sure what the first number you quote means (storage life decreases at a rate less than 0.1 % per month), since that would imply a typical lifetime of 80 years!!! Not even nonrechargable Li batteries have a retention rate even close to that, so I suspect what this number represents is the rate at which the charge retention rate drops off as the battery is charged over and over again (i.e. each subsequent charging attempt results in a charge that's held for a shorter time than the previous one), rather than the retention rate for an individual charge.

So maybe a month is a reasonable length of time to expect a Li-Ion battery to last at reasonably full charge (for Ni based rechargables, make that, perhaps a week). That, of course, is subject to the proviso that the battery be kept at reasonable temperatures (not out in the sun, or in a glove compartment, for example), and not permitted to short out by contacting metal or water. Yeah, it'll probably be good for longer than that, but if it's something you're depending on in an emergency "probably" isn't good enough. "But the manual said..." - famous last words.

That's why I keep both batteries in regular use, and never let one be out of use or the charger for more than a couple days, to best ensure that they'll both be working properly when needed, and to know right away when something goes wrong.