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.