This seems that this is the case with Windows 8 & 8.1, so you are not alone. A piece of hardware stopped working yesterday and finding no solution, I decided to do a reset. I expected the reloading process to take around half an hour, but it in fact took 11 hours before 8.1 was re-installed. Cleaning the hard drive seems to be the problem and the bigger it is, the longer it takes (I have a 1TB hard drive on the laptop I reset).
After 10 hours, the progress showed 28%. This suddenly jumped to 38% and the operating system was re-installed in about half an hour. Looking around the forums, I notice that some people have had to wait 20 hours or more for the reset process to complete. Hope this helps.