So I checked my chain today - it's measuring 12 1/16. I had replaced it in early October, and I haven't been doing much riding - a hair over 1000 miles since then.
Nothing but new cassettes. In fact, this chain has had three brand new cassettes, don't ask. Completely cleaned twice, lubed about every five days or so.
I've got a 105 chain - it was too cheap at PBK to pass up (~$18) - but recommend a chain that'll last.