Re: when to throw out brass ???
Depends. It depends so much, that I'd read into it a little more. Your manual should have some info on brass inspection and such; if not, find another manual. Brass life can vary greatly from caliber to caliber, and even from load to load within that caliber, not to mention difference in tolerances (.223, .308 commercial brass vs. 5.56, 7.62 Lake City brass).
Two of the most common with rifle brass:
Swaging work hardens the brass, so the necks can split if that happens. Some folks re-anneal their case necks to make them last longer. I'd do this for expensive or rare brass, otherwise when enough show up in a lot, recycle them.
Swaging also stretches the case, and it does this most just above the base where that transitions into the case wall. Every time you trim brass, you are really trimming the THICKNESS, not necessarily the length. I mean, you trim it for the length, but that material is coming from the case wall itself, mostly from the bottom. In time, it will deform, get weak there, or even give way or blow out. Case head separations would be common. I'd say that if you've had a few of those from a lot, it is probably time to recycle that lot as well. At best, a case head separation renders the rifle temporarily useless. At worst, it destroys the rifle or even injures or kills the operator through catastrophic release of pressure. There are ways to measure this wear, but you need a special tool. A rule of thumb is the more trimming you do, the less life you have left on that brass.
I'd also find a good ballpark figure for the round you are loading as well. What are other people getting from the same brass, similar bullet weights and charges? This will give you something to go by, as you certainly get more use from a 5.56 case than you do from a .50Beowulf.
Be careful and safe loading!