He who has ears to hear....
How long do you use fired brass. I pick up any brass I find and there is no way to determine how many times they have been loaded. Is there a rule to go by? I know I am cheap

The hand loaders that have spoken before me on this thread speak the truth. But hear ye this.
Many years ago now, Dr Carl Sagan said that "every atom that ever existed still exists".
(Or Atoms do not wear out, nor do they ever get tired)
Brass (copper and zinc atoms) has very little oxidation.
When I was reloading many years ago, I was only interested in 38 special and 357 magnum. Very few hot loads. Very few 357.
So, not being an expert in brass flowing, cases lengthening, hardening, or cracking, some opinions were still gleaned.
Like hitting metals between a hammer and anvil - brass gets harder and perhaps thinner each time it is fired. In low pressure cartridges it is minimal.
The mind game is simple and basic. Draw a one inch square on a piece of paper. Now draw a vertical and horizontal line dividing it into 4 equal parts one half inch one each side. Yes in water pipes and gun barrels 1 inch is 4 times bigger than 1/2 inch.
You can move 4 times more water through a 2 inch pipe than a 1 inch pipe. 4 times more through a 50 caliber than a 25 caliber (at similar pressure).
Stay away from the Kinetic Energy (religious cult) where they square the velocity compounding energy with speed. They will draw you in with there evil flat trajectories. Their exploding prairie dogs at 300 yards.
Their brass flows and gets brittle. It is an abomination requiring a sacrificial beer later on. Much annealing and trimming of cases. Excessive worry. Shun them.