You mean the shelf life of a fully charged battery? In the tablet, probably a few months, at least, until it gets down to the point that you'd better charge it. (Letting it get down to almost 0, which is as low as you can get it, will drastically shorten the life of the battery if you do it more than a few times.)
Out of the device? A really good battery will self-discharge about 1% per month after an initial 5-10% discharge, so about 1-2 years before you have to check it. (But it should be brought to about 50% charge before long-term storage. Then it should be conditioned once it's put back into use.)