Sounds like the LBS owner likes to sell chains. As others have noted, if you change the chain when it reaches a moderate amount of elongation you will prolong the life of the cassette *and* save money on chains.
It's easy to change a chain and "do it right" at any time in your life.
1) Get a chain checker. These tend to over-estimate chain wear, but are very easy to use.
2) Check your chain monthly.
3) When the chain checker indicates wear close to the limit (usually 1%), proceed to...
a) confirm chain elongation with a ruler. If less than 1%, go back to 2)
b) replace the chain yourself, or...
c) have your shop replace it.
I'd be surprised if you don't get more than 2,000 miles out of a chain if you're giving it a little attention once in a while, and your cassette will last for 3 or 4 chain changes or longer.