I save, like, every 30 minutes, or when I finish, because my computer doesn't crash all that often.
But when I'm working with some kind of music program, like TabIt or Finale, I find myself saving compulsively, like every 10 seconds.
I mean, like, I'll even save when it's playing back, knowing full well that I didn't actually make any changes, since, you know, it's playing back. But I'll still be like "This is awesome! I love it! ctrl+s ctrl+s ctrl+s ctrl+s! YES! ctrl+s ctrl+s!!"