Keeping Logs files for how long?

Over the last few weeks we've gotten a bunch of good advice on what
software/hardware to use, in order to capture logs from all sort of
networked devices.

One thing we have not really talked about, is how long do we usually
keep logs. And do we ever back them up?

Any feedback on how you handle the log data, or best practices for
handle log data, would be greatly appreciated.


-Petter