I have a similar problem, seems to be related to 'wipe MFT free space'. It gets up to 28%, then the 'estimated time' just increases exponentially, at 2 or 3 seconds per second. I've noticed the older answers (2012) by moderators on the forum concerning this option seem a bit evasive. Right now I'm running 'wipe free space' with 'wipe mft free space' unchecked, and it seems ok.
BTW, I have a Dell Inspiron 64 bit dual core with a half full 320GB disk rated at 5400rpm and deemed in 'good health' by Piriform Defraggler....
So here is the deal. The timer IS NOT ACCURATE so you have to disregard it. When I run CCleaner, it will get to approx. 97% then it goes through A LOOONNNGGG process before it actually will display the amount of progress, starting at 0 MB of XXX,XXXMB. For instance, "Wipe Free Space 10,000 MB of 455,000 MB." THIS is the accurate reflection of your progress and will EVENTAULLY count up to the entire amount of Free Space and stop.
Be prepared to wait a LOOONNNGGG time once you get to this part of the process although it is going to depend on the amount of Free Space. It took about 26-30 hours of continuous running to complete with 435,000MB of FS. In comparison, I ran this same process on an old computer with only 3,300 MB of Free Space and it took about 5 minutes. Set your computer's "Go to sleep" function at MAX and be prepared to come back and wake it up several times.
Q: Now for anyone reading that can help answer this question, I would appreciate the heck out of it and award to you a prize of 1,000 Internets
(J/K, obscure reference).
The first time I ATTEMPTED to run and stoppped prematurely, it listed ~455,000MB to clean, I abandoned this and tried again the next day. The next attempt that actually finished to completion, it listed only ~435,000 MBS of Free Space.
Just for my own curiosity, I used the TOOL 'Wipe Free Space' on Drive OS(C:), to see if it would be shorter or longer and now it lists only ~422,000MB??? I am not a heavy user of this computer and I am not downloading anything, WHERE is this free space going?
Are files being sequestered somewhere or somehow by MS? Sure it sounds paranoid, but I would trust these douchebags about as far as I could throw 'em. Especially since Windows 8.1 disallows the disabling of Thumbnail Caching and the MS engineer told me "its not advisable" and also that without the "Professional" version that it could not be done.