I backup my thesis files after a day's work, and do it manually. (Of course, I have a simple bash script that does the brute work, but I call the script manually.) I keep copies of the backups on a USB key and on two computers. Other than that, I usually backup my home directory at home five or six times a year.
Over the years, I have learned that the data you will most miss when you lose it is data you have created yourself. I don't bother backing up Ogg Vorbis rips of audio CDs anymore, because first of all, they take up lots of space, and second of all, I can always rip the CD again or borrow it from the library. On the other hand, a Perl script I have written will cease to exist permantently one day if I don't have a backup of it. The manuscript for the greatest novel ever and the photos of your dear cat fall into the same category.