How do I know when to replace a cartridge?
When it stops being enjoyable to listen to.
How many playing hours?
How many miles until you replace a car? It depends, on many things. The cartridge itself, the set up, and usage will also determine how many hours you'll get out of a cartridge. If properly set up and maintained, you should get 2000-3000 hours out of a cartridge. If the VTA, VTF, Azimuth, etc are not set up correctly, you could get much less. Just like you won't get as many miles out of your car if you don't take care of proper maintenance.
When does the sonic begin to go?
When it starts to sound dull or lifeless, then again, it will sound like that if you don't have it set up right too.
Happy New Year,
John