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.