It really depends on how well your bike is jetted. If it's leaking spooge - you've waited too long to repack it - and you are running too rich.
If the jetting is spot on, I'd suggest that somewhere in the 50 hour range you would want to look at the condition of the packing. I usually repack my silencer when I tear-down the bike for major maintenance.
I repack my 2strokes about every 10hrs....blown out packing will effect how the bike runs...I find it doesnt rev as smooth or quick when its blown out....
I have always been able to tell when the packing is getting saturated when the exhaust note develops a 'flat' sound. The feel of the puffs of air from it will also get weak and feel less like individual puffs and more like a blow dryer.
When the jetting is nicely lean, they do burn off the packing, and well, they get loud then.