3500 to 5500 miles per chain for the last half dozen that I've replaced, so if your mileage estimate is right it's probably time. My cassettes last for two or three chains. Were I you, I'd replace the chain now and if it skips on the cogs then replace the cassette.
I've recycled stretched chains onto a worn cog (they can still work together) onto a second bike, but only because I'm really cheap. It's probably a good idea to just replace both the chain and cassette now - but measure it first. No use worrying about less than 1/16" stretch, but more than 1/8" you might as well just write it off.