I have a little under 3,000 mile since I made my last adjustment (March of '09). It's getting a little harder because of my hands and back. I thought the valve adjustment tool my make it easier and faster. The cheapest tool I found was $35.00. For that price I'll use a wrench and screw driver.
Jack says about 5,000 mile. In that case I won't have to do it until late spring, early summer.