RIM tells you to keep for at least 7 days. (But then they also tell you to delete inactive users after 30 days, which I think is a little draconian given overseas travel, maternity leave, etc.)
As for logs, I used to store them indefinitely and back them up to CDROM's when I need to free up drive space. I found that the debug log files for the BES were invaluable in certain situations (many non-BB related) where we needed to track down when emails were sent, were they received, etc.
Unfortunately the log files are just too darn big these days so I purge them after 60 days. A while back I got a request for some info from 62 days previous... oh well couldn't answer that one...