Mine on the 450SL died at 101K. Of course the car was 25 years old at the time... ;-)
Mine gave me so much warning I really couldn't be upset when it conked out and left me on the road. First, it began to make an unusally load buzzing sound while driving. Then, I occasionally could smell gas while driving. The final death throes came when I turned the key to the 2 position, but hadn't yet started the car. Normally, the pump would run for a second or two to pressure up the system. As it expired, the pump would run constantly without the car being started.
It's probably a good idea to swap it out as preventive maintenance, but I wouldn't do it before 100K. Of course, that thinking and a little procrastination is how I ended up with a dead pump...
82 300 SD
77 450 SL (gone)