How many firings before one can expect to experience loose primer pockets on max reloadings of magnum cartridges?
As low as one. Realistically 2-4x if you're really pushing pressure but not to the level of utter stupidity that would blow them in one shot. But that's true for any case, not just magnums. It's not hard to blow primer pockets at all.
One thing to think about is that it's not just the looseness of the pocket, but also what the pressure is doing to the rest of the case. Even if the pockets hold you can shoot pressures that create variance in the brass - matched cases are pretty much the point of buying good brass so why do something to them that defeats that purpose?
Part of why I run big magnums - 300 RUM, 30 Sherman, 26 Nosler, 243 AI (yes this is a stupid over-bore 6mm magnum) and so on, is because they can get the same performance as less excessive cartridges without being pushed hard. 300WM velocity is pedestrian in a 300 RUM. Speed is a byproduct of capacity, if you need more it's readily available to buy, you don't have to push the life out of a cartridge to eek out the last bit of speed - buy the case
and/or barrel length* that hits the speed and the node you want.
I have the same logic with trucks - why beat an F-150 like a rented mule when an F350 will will do it with ease? Maybe I have to buy the XL instead of the fancy King Ranch to keep the prices similar, but what's going to last longer? Sometime run at the redline full time or something that doesn't even sweat doing the work?
* - Barrel length is a big part of hitting nodes. Run the numbers on a 7SAUM in a 22" versus a 7-08 in a 30", look at the velocities barrel times, and you can hit surprisingly similar barrel speed nodes. Not saying one wont be able to hit a faster node the other can't but you can hit similar marks that overlap between the two. If you need short and handy mountain rifle for long shots run a big magnum, if you can hump a long and fat barrel to a blind then a very mild cartridge could work well for you. Multi-variable analysis is time consuming for rifles, but laying out clear goals and building to match them is very worth it in the end.
Even things like accepting that to gain more weight capacity for barrel length to hit a velocity goal you might have to lose some off your own waist comes into play for some of us
<--- me specifically lately
