I was thinking somewhere in the neighborhood of 8 to 10 weeks.
merlyn mentioned above that he's got a new book coming in June that might fit the bill too, but I don't think I'll have that much time to make a decision :(
I find perldoc to be a valuable asset in my class. I personally don't find it as easy for people to read as Learning Perl, but I always make sure to supplement my lectures and discussion with perldoc references. I feel I'd be cheating students out of a large part of their education otherwise :)
MrCromeDome