Thus far we've been keeping courses around indefinitely, and our logs show that students do come back to view the material from time to time (this makes the instructors feel good ).
There's probably no reason delete anything unless you start to run short of disk space.
We also do an off-site backup once a day, and burn two CDs at the end of the course. One CD remains with us, while the other is given to the instructor for his/her own records.
We allow users to remain in the system indefinitely, because we have a "course" that's used as an online community for our alumni.