It's also possible that the time estimate is incorrect. Sometimes it will report a time way longer than it will actually take, but settle to a (more or less) accurate time once things are well underway.
For example, I cloned 3.5TB of data from one of my RAIDs to another just last night. When I started the clone (via Disk Utility's 'restore' option) it said 7 days to complete. Yes, 7 days. When I came back about an hour and a half later, it said 11 hours. That was at 12:30 am. When I awoke at 6:30 am, it was completely finished, including the whole verification stage (where the whole 3.5TB of data is re-read for integrity).
Let it start and run it over night. I bet it finishes far sooner.
One last piece of advice. Make sure you've got a known-good USB cable. A crappy cable might drop you down to USB 1.1 speeds - which really could take 24 hours.
If it really takes that long, there's something wrong with your target drive.