It was a local hard disk and just from one folder to another. For some bizarre reason it takes about 2 hours for Vista's Explorer (I'm specifying this since we can use other file browsers) to copy 98 MEGABYTES from one folder to another.

So if we take your 100GB as 1000X more data and if Vista stumbles like it did there AND you use Explorer then that's 20,000 hours or 833 days or about 2 years.

That's the longest possible time it may take on Vista.

Now I did clone an 120GB drive from/to USB 2.0 and an internal drive in about 2 hours so this sets the range of performance and gives me my answer.

-> From 2 hours to 2.28 years.