How long can a device be out of communication before the BES assumes it isn't coming back?

I don't think theres any limit [note think not know] ..if that PIN of the device is still on the BES and there's a valid encryption key in place then it should be able to send /receive mails, even if been 'off air' for a while..

you might be better off asking in besadmin conrner however

Thanks for the reply, I'll ask over in the other forum as well.