From Wolff's website:
4. How often should I change my springs?
Wolff Gunsprings are made with the highest grade materials and workmanship. Most Wolff [recoil] springs will remain stable for many thousands of rounds. The performance of your gun is the best indicator of when a spring needs to be replaced. Factors such as increasing ejection distance, improper ejection and/or breaching, lighter hammer indents on primers, misfires, poor cartridge feeding from magazines, frequent jams, stove pipes and other malfunctions are all possible indications of fatigued springs or improper springs. Springs that are subject to higher stress applications such as magazine springs, striker springs and recoil springs will require more frequent replacement than other less stressed springs. Most Wolff recoil springs should be capable of 3000-5000 rounds minimum before changing is required. Some recoil springs in compact pistols, especially where dual springs are replaced by a single spring may require changing after 750 - 1500 rounds. Changes in your firearm's performance are one of the best indicators that a change is needed.
I would say a PPK/S as well as a PPK qualifies as "compact pistol". If you are using your Walther for personal defense and not just as a range gun, I would err on the side of less rather than more. I'd change between 750-1000 rounds. Now watch how many people post "I've fired a hundred zillion rounds through my Walther and the spring is still fine"
Dep