You can use dd. The following will need to be customized! Run:
dd if=/dev/zero of=/dev/sda bs=512 skip=0
Namely, you need to customize the drive name. In this example, I'm treating the drive name as /dev/sda, which is the first disk using SCSI, SATA, or some other /dev/sd standard.
Then, when you're done doing your partial wipe for the day, press Ctrl-T (hmm... that didn't seem to work for me, but other people said it did work for them) or send an appropriate signal (see Caleb's answer to kivetros's question about seeing dd status). Or press Ctrl-C to just quit dd, showing status reports. Test this early, so you know the right procedure for your operating system, and don't accidentally terminate dd by using a procedure that isn't right for whatever operating system you use. For instance, doing the right process for BSD when on Linux, or vice versa, will result in the "kill" command living up to its name. (Sending the right signal can result in the "kill" command appearing to just do what it really does: send a signal. Despite its name, the "kill" command's main functionality is just to send a signal.)
The downsides to this approach are that you'll need to run dd (which doesn't come pre-installed on Microsoft Windows) and may need to learn a bit of Unix (to identify the drive name, and to understand how to send a signal). The upside is that you can make partial progress, record the numbers related to your progress, and then abort whenever you want, offering you flexibility (to stop at any time) and maximum progress (not stopping half an hour early just because you tried to time it to finish). It will also be able to get every single sector on the drive, unlike if you try to use partitions.
After you are done for the day, get a status report for dd. e.g.:
8279963+0 records in
8279962+0 records out
4239340544 bytes (4.2 GB) copied, 8.51097 s, 498 MB/s
Then use that information to increase the "skip" by the amount that was successfully copied. (Look for the blocks that got written out.) For example:
dd if=/dev/zero of=/dev/sda bs=512 skip=8279962
Then, if the next day, you get:
4386149+0 records in
4386148+0 records out
2245707776 bytes transferred in 2.608 secs (860766103 bytes/sec)
dd if=/dev/zero of=/dev/sda bs=512 skip=12666110
(Just add the "records out" to the prior "skip".)
Hopefully this next paragraph is intuitively understood, and is unnecessary, but just in case:
Just to re-iterate: customize the name of the device so that you're not writing to the wrong drive! Wiping is a very dangerous procedure, in that it destroys the accessibility of data, so be careful with any command. You probably needn't worry so much if you're wiping the only writable drive in a system, but you absolutely do need to be very careful if your system has multiple drives and you'll only intending to wipe to a specific one of those drives. Completely wiping the wrong drive will render the unfortunately-wiped drive to be irrecoverable with commonly performable, easy data recovery techniques, and even a partial wipe starting at the beginning of a partition will render some data to be essentially irrecoverable, and often render the other data to be very challenging to recover. Be careful!
(The reason that my "dd" output looks a bit different is that I got samples from different operating systems. Don't worry about minor differences in the third line.)