I have an iPod touch 4th generation 8 GB. I left my iPod recharging a long time after it was done charging. Will it do something bad to my iPod touch?
Not at all. You can leave it connected to the charger as long as you want.
You may want to take a moment and check out several support articles ...
Apple - Batteries - iPod
iPod: About the battery charge status icon
Accurately determine your iPod battery life
Retrieving data ...