What really matters is the weather & road conditions. Whether it's 5 miles or 25 miles in the rain, I'll have to clean my chain. By "clean" I mean, I lube each link and run the chain through a rag for a long time. Theoretically that is "until the chain runs through the rag cleanly" but in reality it is "until I'm tired of turning the pedal by hand".

I was very proud when I got my new cassette because the mechanic said my diligent chain-cleaning really prolonged the life of the old cassette (4500 miles). He said some people come in after 1000 miles and their cassette is already trashed. Ultimately what matters is how smoothly the chain goes onto different cogs, and whether it is wearing down your cassette.