I sometimes leave my computer running when I need to download huge files (never mind what sort of files ;-) These downloads can take anywhere from a couple of hours to six hours to complete. But I don't want to leave the computer running longer than that.
Solution: timed shutdown.
The command is ridiculously simple:
sudo shutdown -h +180
for example, if I want the computer to shut down in 180 minutes' -- or three hours' -- time. Just enough to finish the download. Make sure to keep the console at which you issued the command open.
Take note that the computer will only notify you at the :00 and :30 marks (unless you're near the actual shutdown time, in which case it alerts you in multiples of five minutes).
Additional tip: shut down the monitor.