Teresa,

It's not mileage but how and where you ride that make the difference. A clean chain (see above) stays in good shape longer than a sandy or non-lubricated one.

When should you change?
Sheldon and quite a few others measures "stretch" http://sheldonbrown.com/chains.html

The mechanics at my repair shop use another simpler technique: place the chain on the large chainring and pull gently the chain via the midpoint. If it is so stretched you can see air between the pulled chain and the chainring, then it is overstretched and should be changed. Otherwise, it's ok.
BTW, I have also heard that trick quite a few times from other French-speaking sources, be it from Québec or from France.

Regards,