How long does it take to zero a 1Tb drive?

About 10 hours for a USB2 external drive:

root@bobcat:~# time dd if=/dev/zero of=/dev/disk/by-id/usb-TOSHIBA_MK1059GSM_0FA6FFFFFFFF-0:0-part1 bs=4M

dd: writing `/dev/disk/by-id/usb-TOSHIBA_MK1059GSM_0FA6FFFFFFFF-0:0-part1': No space left on device
238467+0 records in
238466+0 records out
1000202241024 bytes (1.0 TB) copied, 37410.6 s, 26.7 MB/s

real    623m30.583s
user    0m3.416s
sys     77m19.798s