Where the rate starts to fall
Almost everyone opens a thirty-second run faster than they finish it. Forearm tension is the limit, not finger speed, and tension is what jitter and butterfly both spend to buy their rate. The result is a per-second chart that slopes down, and the slope is the reading that matters more than the total.
If your last third is within ten per cent of your first, the rate is sustainable and the sixty-second interval will confirm it. If it has fallen by a quarter, the number you have been quoting from short tests is a peak, not a rate.