- Keep your chain clean and lubed.
- Check for wear with a ruler (as fietsbob described above) or use a chain checker.
- When the chain checker says it's at 1%, or your ruler says 12 1/8", replace the chain.
That's what a reputable LBS should say.
I agree with that except that I personally wouldn't let a chain get to the 1/8" point, by then you're probably wearing the cassette and the new chain will wear faster as a result.
Chains can wear out in 500 miles or less, but that's a special case of mud and mountain bike racing. Even riding straight through winter's slush, grit and salt, I get better than 1,000 miles on my winter chain. In the three-seasons, about 2,500. And I'm pretty hard on chains. I ride with people who get twice the mileage I do.
Really? I've never seen a 9 or 10 speed chain last 5000 miles. Anyway, the best advice the OP can take is to clean frequently and check for wear as he approaches 1000 miles, and every couple of hundred thereafter.