How much is a "lot of use...?"
No exact figure, but substantially less than a comparable steel gun, or even an aluminum one. Zamak items are made for one reason: They can be cast cheaply and give a reasonable service life to such things as window cranks, power tool and fan frames, etc.
"...and how "quickly" is quickly?"
Again, it depends; If you have impurities in the castings, pretty quickly. Likewise, if constant battering causes work hardening of, for example, your slide, it's also gonna crack pretty quickly.
Bottom line: Manufacturers are using Zamak because it is cheap and quick to work with. They make the big bucks (as I stated), and you get something to play with for a while, but it is still a matter of when, not if, something "bad" is gonna happen to it. I've seen it more than once in my career, including a Zamak gun that was so badly corroded, it literally crumbled to pieces when the barrel was tapped on a carpeted sales counter in the shop where I did my repair work. If the poor old woman who owned that gun had fired it, it would have been a hand grenade. Another pot metal special came in the shop that was corroded shut with a live round in the chamber. Like a monumental dumbass, another one of the gunsmiths tried to fire it to dislodge the slide, and succeeded in having it come apart like a grenade and ended up with fragments of zinc in his hand. Luckily, no one else was hurt in the incident.
Another pot metal scam has been perpetrated for years by Marlin. The feed ramp on the Marlin/Glenfield Model 60s is Zamak and would wear out in short order. They were easy to fix, but the catch was that Marlin was constantly making "engineering" changes, so you had to buy a new bolt assembly to go with the "upgraded" Zamak feed block.
To you and anyone else who wants to buy a cheap toy to play with, go ahead, but don't delude yourself into thinking that your gun is going to last a lifetime. If you're lucky, it will wear out much more quickly and become unusable and economically unfeasible to fix without doing any harm to you, as opposed to a comparable steel/aluminum (and now, much to my disgust, plastic) gun.
What floors me about the current craze for Zamak guns is that even steel firearms have "blown up." Guns are, by design, machines that employ principles that operate on the edge of physics and metallurgy. Why would anyone want a gun made out an inferior metal?
All the above are just my opinion of Zamak guns, and my mind has yet to be changed about them based on what I have seen in the past.
As always: YMMV
