Breakfast - 50 rounds of fried 158 gr. HP, side of 125 grainers
Lunch - 50 rounds of 158 SWCGC, 15.0 grains 2400
Dinner - appetizer of one box of 125 HP, main course of two boxes of 180 JSP.
Okay, so I'm a sucker for overused phrases, too.
Basically, I'd call a steady diet shooting at least one or two 50 round boxes of full power .357 Magnum rounds every time you shoot. Depending on the frequency of shooting - daily, weekly, monthly, once a year on your birthday - a revolver could last a lifetime or start to develop end shake in a year or two.
But instead of trying to absolutely quantify "number of rounds = worn out" which varies from piece to piece I'll put it another way. Based on several examples from gun show tables I'd definitely want to examine an older Model 19 in person to check for signs of end shake or other wear, whereas I'd have few qualms buying a used Security Six from the internet as long as the external condition appeared okay.
Heck, I shot my 1981 vintage Model 586 enough to develop some endshake after about 20 years. Sent it back to S&W and they fixed it up nice and tight again.