Bicycle Mechanics - "When-to" Cassette Replacement

Does anyone have any 'when-to' rules of thumb regarding when a cassette needs replaced? There are plenty of 'how-to' articles out there, but I'd be interested to hear 'when' as well - i.e., from a 'old one is worn out' perspective or similar thoughts.

I replace my chains when two rivets are spaced 12 1/16" apart. Cassettes will typically outlive several chains if they're replaced that frequently.

If, after replacing the chain, I get "skipping" on a favorite cog, it's time for a new cassette.

Does anyone have any 'when-to' rules of thumb regarding when a cassette needs replaced? There are plenty of 'how-to' articles out there, but I'd be interested to hear 'when' as well - i.e., from a 'old one is worn out' perspective or similar thoughts.

Tough question to quantify.

1. Cassette definitely gets replaces when a chain stretches so much as to ruin the profile of the cassette cogs such that a new chain repeatedly skips on the cogs. See Sheldon Brown's article, about 2/3 of the way down: http://sheldonbrown.com/chains.html. For example, if the measures greater than 12-1/8", your cassette is probably ruined.

2. As RetroGrouch says, replacing the chain before the 12 1/16" stretch will prolong the life of the cassette. But wear of the cogs will always occur with use. In my own experience, the chain to cassette ratio is about three. I'm pretty meticulous about lubing and maintaining the chain. I ride mtb so the drive train is exposed to lots of nasties. And of course, YMMV.

OK, thanks very much. This is chain 2 for the cassette in question. I, too, am fairly meticulous about keeping the chain relatively clean and lubed, and I don't have a need to ride in nasty weather very often. I'd suspect I am getting good wear based on those factors. I live in a very flat area, and spend a lot of time in a few particular cogs so that'll probably be the wear mechanism. To complicate things, I was temporarily in a relatively hilly location this fall, so I had my 'granny cassette' (no offense to anyone intended) installed. This would probably tend to extend my normal cassette's life but makes the three-chain rule a little harder to track.