When the Tablet is connected to the charger, the displayed remaining charging time is completely wrong.
Same on my Gigaset GS6 Pro, but I am not that bothered and just let it charge.
It is all over… two days ago it got stuck on 3 hours where it never takes that long, and kept that prediction until 100% 
Out of curiosity do you reboot the table often. Have you also tried charging while unit is off?