Why is Windows.old still on my Venue 8 Pro?

Yep.  But perhaps the timer is not as definite and accurate as that.

I forgot I posted this question.  I think about a week after I upgraded, it asked me if I wanted to delete the old files, but I wasn't ready to do that yet.

I'll give it a few more weeks. If it's still there, I'll try the disk cleanup procedure to do it myself.

And you'll love all your new-found space!   :emotion-1:

I used the Disk Cleanup procedure to remove the restore files from the previous installation of Windows 8.1 but it did not delete Windows.old. While Disk Cleanup deleted most of the files, 3 files remained in a sub folder called FileRepository that proved difficult to delete. In addition, a folder call TEMP1234 owned by TrustedInstaller was created by the Windows 10 Upgrade and cannot be deleted using Disk Cleanup.

Were you then able to delete the stubborn files manually?  If not, hopefully, an automatic Windows clean-up routine will do so.

The 3 stubborn files in the FileRepository sub folder of Windows.old turned out to be exact duplicates of files in Windows\System32\drivers. Someone posted a procedure (if I recall correctly) that involved moving the duplicate files to Windows\System32\drivers then I could delete the folder. I believe the issue is that the files were associated with files active in the operating system in which case Windows clean-up routines will not work, but you could boot-up another OS from USB.

Thanks--hopefully, Windows' own file clean-up routine handles matters like that. 

Just to report that when I went to my DV8P early this morning, I found that I had 4+ GBs of extra space on my tablet (!):  Windows automatically had deleted my Win8.1 rollback files from my July 28 upgrade to Win10, 3-1/2 days after the end of the 1-month rollback option period.  Perhaps it was the rollover to September that kicked Windows into deleting the rollback files earlier today in the wee hours.

Similar to what is noted above, I still have leftover 5 small .sys files (under a quarter of 1 MB in total) in a still-existing Windows.old folder on my C drive--think I'll just let Windows handle them in due course.

An extra 4+ GBs of space on the tablet's built-in memory--haven't seen that on my DV8P in well over a year, or two!  I feel so, I don't know, clean!  Wonder what I should do to celebrate--download a few movies and fill it up?   :emotion-1:  

I think I might have been a month off in reporting that I updated on 6/27.  That might have been my desktop update (which doesn't show windows.old).  I'm expecting my tablet to be cleared of the update files any day now.