Realize that once you work on it, you have voided any warranty on the workmanship of the install. If you aren't confident that you can diagnose and fix the problem on your own, take it back to the shop that did the install and they should fix it under the original install agreement. Checking the rear derailleur adjustment and assuring that the chain is the proper length are normal parts of an installation. A good mechanic won't let a bike leave the shop unless everything he/she touched is in good working order.
Clean, lubed and in proper adjustment, a RD will last a long, long time. The only things that are likely to wear (usually in much more than 2,000 miles) are the two small sprockets and those are easily replaced. I've fixed up bikes that are 30+ years old, rusty and badly neglected. Almost always I've been able to use the RD after cleaning and lubing it. The only times I've replaced an RD has been for a substantial upgrade or when one has been damaged, never for wear. I'm sure there are riders who have actually worn out an RD, but it should take a lot of miles.
Last edited by Myosmith; 08-19-12 at 08:08 PM.