whenever my gun stops working. for me it was once.
but seriously, you need to know your gear. how dirty does the gun run? i would take a look at the gunk elsewhere in your gun. if gunk builds up behind hte extractor hook, it can slip off the brass and fail to extract. if it builds in the channel, it'll weaken the grip on the rim. at the most extreme, it would bind and not go into battery (similar to when you drop in an extractor spring that binds).
just for kicks, here's my thread from when i cleaned the gun feb 25th...
http://www.brianenos.com/forums/index.php?showtopic=146990&st=0