I keep an ammo can of 223 loads that I use for plinking - cans, rocks, armadillos, etc. I basically worked up a load that all of my ARs shoot pretty well (69 Sierra over 25.3 grains of R15) and keep the ammo on hand, reloading as I run low.
I full length size using a Redding bushing die.
I do no case sorting, so there's Winchester, Remington, LC all mixed together. No tracking of loads, so some brass may have 2-3 loads; some more, possibly many more. I've trimmed on occasion, annealed when I felt like it, on no schedule. I've not noticed any loose primer pockets, no splits, etc.
At what point should I just throw out and start over? If there is a case failure, what exactly will that entail? Any danger to me or my rifles?
The .223/5.56 cases below were measured for Rockwell hardness in the base of the case and were from a heated discussion at AccurateShooter.com
and posting #83 by CatShooter.
Raised Ring at Base of .223 case?
The actual measurements were (.062"x100kg, Rockwell "B")
LC 2008 = 96
Lapua 223 Match = 86
Winchester 223 = 69
Remington "R-P" = 49
Lake City 5.56 cases and civilian contract military ammunition are made to higher quality and hardness standards, this is because these cases are shot in larger diameter and longer headspaced firearms.
The answer to your question "When to pitch .223 brass" depends on the brass used, your chamber, your sizing die and the amount of shoulder bump and head clearance the cartridge has in the chamber. It also depends if your shooting a bolt action rifle with a SAAMI chamber or a military type AR15 rifle with a chamber larger in diameter and headspaced longer.
Your shoulder bump controls the amount of head clearance and the distance the case must stretch to meet the bolt face when fired. If the case is over resized the shoulder is pushed back further than needed, the case stretches and thins and you have case head separations.
Below is an animated image of a cartridge case being fired and stretching to meet the bolt face. Normally for a bolt action shoulder bump is .001 to .002 and for a semi-auto it is .003 to .004,this minimal amount of shoulder bump will extend your case life.
Below is an image of a case being full length resized, pay attention to the dotted blue, red and green lines and in the center of the image is "shoulder setback" or shoulder bump. Bottom line, the further you push the shoulder back the more it can stretch and the sooner the case will fail.
Below is from Reloader Magazine, the .308 cases below were fired in a new Savage .308 rifle and full length resized with the die making hard contact with the shell holder and the press reaching cam over. The problem with this information is the rifles headspace setting was not mentioned and therefore the head clearance was unknown. So the information below is a "ball park" guesstimate of general case life longevity.
I reload for my two sons rifles and hand guns and don't want to have them say "dads ammo is junk and falls apart" so each case is checked with a RCBS case mastering gauge for thinning in the base web area
And I'm knocking on wood but in 46 years of reloading I have never had a case head separation.
So to truly answer your question "When to pitch 223 brass?" is up to you and your reloading techniques, and your quality control of your brass.