Actually, putting it in Airplane mode is how I charge mine every night as Meg suggested. I do not need my device during the night, since I still have a home phone. Then in the morning when I get up, I take it out of Airplane mode and the notifications begin! I don't recall the time period either, but powering off and/or resetting the device requires the passcode to enter the device again. I think the time period of inactivity is something like 48 hours. I'm looking for it now.
EDIT: An excerpt from this support document. Use Touch ID on iPhone and iPad - Apple Support
If Touch ID doesn't recognize your finger, you'll be asked to try again. After three attempts, you'll be given the option to enter your passcode. After two more tries, you'll need to enter your passcode. In addition, you'll need to enter your passcode to unlock your iPhone or iPad in these cases:
- After restarting your device
- When more than 48 hours have elapsed from the last time you unlocked your device
- To enter the Touch ID & Passcode setting