There is no magic mileage formula that can calculate when you should replace your chain. But, you can measure your chain and know for certain when it is time to do this critical preventative maintenance to your bike. Former team Highroad mechanic Greg O'Keeffe shows how you can use a tape measure to check for chain wear.
Shop for deals on new chains at Art's Cyclery. http://www.artscyclery.com/Road_Bike_Chains/catpage-RDMCHAINS.html?crumb=RDMCOMPS