Longer than most people think. I have 12V power tools with Li-ion battery packs that are 10 years old now. The haven't been used a lot, so the batteries didn't get many cycles, and they're all still usable.How long is the shelf life of those batteries???
Now that's clearly a qualitative assessment, not a quantitative one, so about 1 year and a half ago, I decided to do a quantitative test. I bought a batch of brand-new VTC5As, measured capacity and put them in storage for a year under different conditions. Then I measured their capacity again:
Room temp, fully charged: 3.2% loss
Room temp, ~50% charged: 2.3% loss
Fridge, ~50%^ charged: 1.1% loss
Freezer, ~50% charged: 0.9% loss
So you would think keeping them in the freezer is best. Not so fast. Those cells had a slightly greater increase in internal resistance, so right now, I'm recommending keeping cells in the fridge.
All of them have been back in their respective storage conditions for another half a year. Come July when they've been in storage for two years, I will test them again, as well as a set of P26As that got added to the experiment last summer.