I've been replacing my chain about every 800 -1100 miles. But, I often ride through muddy trails, type of stones and it's dust used in MD and PA creates a real mess. Along with occasionally going through streams and such, has been wearing out my chains easily. I've replaced the rear cassette with the Shimano HG400 and the SRAM PC 971 chain and it shifts really nice now.
Have you checked the chain with a chain checker yet?