Generally the same rule applies to all lithium ion batteries.
Generally if your going to not use it for 3 months or more it is recommended to leave it around 40% then remove it from your device.
Since iPods have non-removable batteries, that's not an option.
Make sure you poweroff the iPod touch and not just lock the screen.
Regardless, if it's not used for 3 months you might be at 0%.