Look, whatever folk are saying, this is a change in operation...
I've had an iPhone 6 for a year, using the finger print reader to unlock. As I don't leave this for anywhere like 48 hours before unlocking I never saw this 48 hour lockout message until month or so... Now I see it quite often... As others have posted, the setup for this says you need to use the passcode if not unlocked for more than 48 hours - it's not specified if this is unlock by password or fingerprint reader. I know in the last nine months I NEVER saw this, now I do, so something has changed. To me it sounds like a bug... and from what I can see not connected to unlocking by passcode or fingerpimt reader. And yes, I know you need to use the passcode also if the device is restarted or you enter the Touch ID and Passcode setup.
Oh, and by the way - I now see this issue on my 9 month old iPad Air 2 - and now it's iPad Pro replacement - the only common item, all running the same recent iOS s/w.
Why can't Apple just comment...