From my informal testing, it will auto-discharge a battery that is >65% full down to 65%, after 10 days of no activity. No activity means you never touch the button to turn it on, or even check the charge. It takes a little over a full day to discharge a full battery down to 100%.
If you are in the habit of storing your batteries full (not recommended, but I do it sometimes too) and don't want it to auto-discharge, just tap the power button every week or so to reset the 10-day timer.