Being pragmatic, as much as you can comfortably afford when the prices are low. Supplies are ample, and we're very unlikely to see prices lower than what they are right now, so it's a good time to buy. Don't put yourself in the poor house, and definitely don't use credit unless you can pay it off right away. You're not saving a thing if you buy the ammo on a card and wind up paying more in interest than you would if you'd bought the ammo at a higher price. And paying cash for the ammunition but then having to use credit for other expenses is no different. People get themselves in a lot of trouble spending plastic money on things because it's a great sale/deal, not thinking about the interest that winds up costing them more than they'd have paid to buy the item at full price with cash at a later date.
For a long time, I've been buying how ever much I could comfortably afford when the prices were what I consider very reasonable. When Wally world was still carrying the ZQI M855, I'd buy at least 2 or 3 boxes every trip, sometimes more. After tax, the stuff was $0.36/rd, which is slightly more than you'd pay buying case quantities at the right time, but it's less painful at $20 here and $50 there than $330 at once. Didn't take very long before I had a case worth, which I put into a water tight container and then started on the next one. Once they dropped the M855, I began doing the same with the .308. Much like a savings account, where it's difficult to deposit an entire paycheck a couple times a year, but taking 5% or 6% out of each check, you barely notice, with the same results.