When I purchased my car it had approx. 58K on the clock, I definitely had to change all of them by then. Not only were they starting to squeak upon breaking, but when I finally got around to changing them out I noticed that the brake light sensors on each of the pads were already starting to show. However, I wouldn't recommend waiting until they squeak to change them out.
Also, check your service manual to see if that's a part of your scheduled maintainence for 30K. If you don't have the cash at the moment, you may still have a couple thousand miles to buy you some time.
Better yet, if you want a DIY, here's a good link:
http://www.bombaydigital.com/boxster/projects/brakes/change_rotors/