I had an ACE. Lasted about 3 or 3 1/2 years. It stopped being able to track inner grooves and complex passages. It was pretty obvious as I am really sensitive to groove distortion. I'm sure it's not a sudden thing though. It's not like it sounds perfect on record and then, bam it goes bad. It will be gradual, but eventually it will get bad enough that you will hear it. That's just my humble experience with just that one cartridges. All others have been replaced before they went bad...within 2 or 3 years.
When to replace cartridge or stylus?
I have a Benz Wood H2 cartridge that I bought used about 2 years ago which supposedly had low hours on it. It sounds just fantastic in my setup. How do you know when the stylus or entire cartridge needs to be replaced? Will it slowly just degrade over time or will it something that will be obvious. If it is a slow decay then I would imagine that it would be hard to realize its performance faultering without A/B'ing with a new one. I certainly dont count the hours I play on it as some do. Also, with a Benz, is it recommeded to replace the whole cart or just have it re-tipped. Re-tipping can save you some bucks.