I retired three years ago today. That being 2010, back in 2006 (after Katrina) I started stocking up for retirement, both commercially loaded ammo, and reloading components. In those days, brass was still available on Ebay, and I literally got enough .223, .38/.357 and 9mm brass to last a lifetime. I slowed down on stocking up on "bricks" of 22LR somewhere around the 20K mark. I don't actually shoot my 22's much, so that will probably last, and leave some for the heirs to pick over. Acquired a Mosin, so had to get a couple tins of x54R, of course, along with a good supply of Monarch SP back when Academy was selling it $6 or $7 a box/20. Then came the SKS, and the need to stock up on x39. So by the time I retired in 2010, I thought I was done buying ammo (or components for reloading).
Then a year ago I bought a Ruger GSR, and more recently a CMP M1 in .308. So, of course, I had to stock up on .308, right during the latest craziness. Finding it wasn't so hard. The hard part was paying for it! You know, that part about being retired and living on a "fixed income."
Retired or not, my rule of thumb is 1K of rounds per center-fire firearm minimum, 5K if semi-auto or used regularly, and for 22 a minimum of 20K. These minimums are for commercially loaded ammo. Reloading components for calibers I reload (.38/.357, 9mm, .223, and I've tooled up for .308) are 1K of brass (at least; for the pistol calibers, I have a lot more), with enough powder, bullets and primers for 5K.
I only shoot occasionally.