Depends on the gun... Shock buffers run forever in Shadow 2s with light competition loads. In CZ Tactical Sports 40SW, however, they last for 600rds at best. I used to replace them every day during a multiple day major match or during a long practice day. I would still use them because I would be breaking everything on my CZs without them. I was breaking slide stops every 2K, both rear and front sights multiple times (At 10K intervals), CGW firing pins and extractors (At 10K as well), and had to reapply the blue loktite constantly on every damn screw. Then I started using the shock buffers and all those issues stopped or became less frequent.
In my real Limited 40SW guns they last for a long time even with lighter recoil springs. The buffer I am currently running is at 7K or more, I stopped counting.