Where are you riding ... is it dusty, gritty, sandy, muddy ..... I've only just learnt how much crap you can pick up in the summer with too much wet lube it was literally caked on the chain, cassette and jockey wheels and I'm sure it'd shorten it's life dramatically if I hadn't of realised and cleaned it off.
Some people set a regime of using the 3 chains ... run the 1st 2 until they've stretched to 0.5 and the 3rd until it's stretched to 0.75 and then go back and use the 1st two until they hit 0.75 .... supposedly this gives you the max usage out of your chains and cassette.