When I pick up a used bike, I like to replace the bars as soon as I find some I like on sale.
After that replacement interval depends on number of crashes and their severity. I've had my 3T bars for about 10 years now and thinking about some new ones. They've probably been in about 7 or 8 mild crashes/laydowns.
I've never broken a bar but saw a guy break one at a MTB race and it wasn't pretty.