MacWorld discovered the change, documented in Apple's iOS Security Guide, after being asked a question by a user. The security measure disables Touch ID if "The passcode has not been used to unlock the device in the last six days and Touch ID has not unlocked the device in the last eight hours."
Users (including this reporter) began noticing this change in the last several weeks, even though an Apple spokesperson says it was added in the first release of iOS 9. However, a bullet point describing this restriction only appeared in the iOS Security Guide on May 12, 2016, according to the guide’s internal PDF timestamp. Apple declined to explain the rationale for this restriction.
It's unclear why Apple chose six days and eight hours but the change should increase security and perhaps protect users from court orders forcing them to unlock their devices using Touch ID. Recently, a judge ordered a Los Angeles area woman to use her fingerprint to unlock her iPhone for the FBI. Many feel that the act of compelling a person to unlock their iPhone via fingerprint breaches their 5th Amendment rights to protection against self-incrimination.
Here's the full list of situations in which a passcode is required...
Touch ID and passcodes
To use Touch ID, users must set up their device so that a passcode is required to unlock
it. When Touch ID scans and recognizes an enrolled fingerprint, the device unlocks
without asking for the device passcode.
The passcode can always be used instead of
Touch ID, and it’s still required under the following circumstances:
• The device has just been turned on or restarted.
• The device has not been unlocked for more than 48 hours
• The passcode has not been used to unlock the device in the last six days and Touch ID has not unlocked the device in the last eight hours.
• The device has received a remote lock command.
• After five unsuccessful attempts to match a fingerprint.
• When setting up or enrolling new fingers with Touch ID.
Read More [via MacWorld]