I don't think there is a specific number. A lot of users have had their BBs for 2-4 years. The only thing that I've noticed is that the times between charges become more frequent.
I don't know about BB's, but I have a 6 year old Sony Clie with a lithium ion battery (same as BB's) that is still chugging along. Given the rapid advances and changes in hand held electronics, my guess is your device will become obsolete before the battery does.
