I had an early Model 642 crack like that. I had bought it new in April, 1991, and shot it regularly, often qualifying it with my department's Plus-P ammo as well as my own practice. It had several thousand rounds through it. It cracked in July, 2011, me finding it when cleaning it after a range session of 100 rounds. So, it may happen soon after use begins, or before use begins, or after a lot of shooting, or any time in between.
It was working fine when it cracked, and had I not found it while cleaing it, who knows how many more rounds it may have fired before becoming noticable or stopped working.
That said, I had a spare already bought sitting in the safe. S&W promptly replaced it. If either of these two crack, I will replace it with another Model 642.
In my mind, it is the perfect combination of concealable size and weight, controllable weight, coupled with an adequate cartridge and outstanding mechanical reliability.