I was backing up to DVD-R, but I changed over to firewire drives about a year ago. They're so affordable now and so much easier to deal with (faster write and read, take up less space), and the capacity is large enough that you can use the same drive for quite some time.
Most archivists say you should transfer to a new hard drive after a number of years, but by the time that's an issue, the projects will be old enough that I won't feel responsible to keep them. It's also said that you should spin it up at least once a year, but I spin the current drive up once a month to add the new sessions to it, so that's not a problem. On my full backup drive, I will spin it up a year from it's last backup just to be safe.
I figure that I should keep things for a year minimum, and past that I'll hold on to it if I have room, but I'm not going to take extraordinary measures to keep it after a year. Practically speaking, however, most everything stays around longer than that.