About 1.75 TB. I got the machine back from Apple yesterday with a new disc, the current OS, and nothing else. I booted it, it saw my Time Capsule, and in just a few clicks and 18 hours, it's back to normal. It reported a transfer rate varying between 8 and 25 MB/second, I'm not sure why it would be that low. My broadband is faster. Maybe I should have remembered to plug in the Ethernet before it started over Wifi.
Having said that I had a backup that ran for a comparable time over USB2 last week. I had resized my hard drive from 3/4 TB to 1TB and told windows to do a backup to the same drive I had used to do the restore from the smaller image. Part of the problem was I forgot to disable Malwarebytes until 24 hours had passed uneventfully (as in the "end of backup" event hadn't fired), but I think that there was some sort of messing around comparing block usage with bit maps too. I should have used a clean USB drive from the get-go.
[rab] The backup was made with Time Machine, but the restore process was handled by the Migration Assistant. Apart from whatever speed issues there may be, it seems to me that Time Machine works the way that all backups should, ever since disks superseded magnetic tape. You aren't limited to just doing a full restore, you can look at the backup and see all the individual files and access them just like they're ordinary files on a disc, which of course they are, not hidden in some opaque archive format. Automatic hourly backups meant that without having to think about it, when the disk failed I lost no data. Do modern backup solutions for other systems provide this level of does-it-right-ness?
Discs haven't superseded mag tape. The cost factor is with the disc, but in streaming mode tape can beat a spinning disc in speed of writes. Not only that, you might be surprised at how robust the tape market has become since defense in depth backup strategies were developed in the wake of cheap,reliable discs. Of course, it is all about to change again as Flash becomes cheaper and more reliable.