I replace the chain on my commuter once a year (3K miles or so) while checking it (with a ruler) a couple times during the season after cleaning. At the end of the season it's usually well worn. I'll change the cassette every 2-4 years depending on how many miles I've ridden just because. I've never had chain skip on the cassette. I also replace the inner cable yearly because I shift a lot and my STI's are hard on cables. Mid range SRAM chain is about $20 and a 105 cassette $30, Ultegra $40-50, so not all that much cheaper.