I would suggest you never back up it. I would advice you to be ready to re-install all software from scratch, starting from re-formatting of disks. If you're not able to do this at any time, you're at risk in principle. You only need to backup you own work.
You should do all your works under some revision control system, as Albert already suggested. Please see this discussion:
Revision control systems, which to choose from?[
^].
I would advice you do all your works under revision control system, even all small works.
—SA