Of course, it also depends how you're zeroing out the drive. Writing just a zero to every area is faster than, say, the more secure wipe method of writing a 0, erase it, write a 1, erase it, write a 0, and check that the write reads 0, and repeat for 32 passes.
It might just be easier to use something like Partition Magic to safely delete the partition, create a new, blank, unformatted one in the empty space, image the partition, then safely delete the new partition, create a 2nd new blank, unformatted one, and then restore the imaged partition, and safely delete it. While it seems like a lot of useless steps, the partition sizes will be small since hardly any data will be used, thus, making the restores, creations, and deletes quick. Plus, creating and deleting multiple partitions to the same area should make it very difficult to recover anything through means beyond electron tunnelling.