⚠ Someone's selling you a mobile proxy that "changes IP every second". Is that even a real thing? Short answer, no, and it helps to split "how fast" into three separate questions.
One: rotation floor, the fastest you can force a change, which is a radio detach/reattach plus your command, a few seconds at best. Two: how long a line holds one IP if you leave it alone, that's carrier policy, minutes to hours. Three: whether the carrier swaps your IP on its own, which on an active line we basically never see on a seconds scale. Those tidy 60/300/600-second spikes people "observe"? That's their own rotation timer, not the network.
iProxy runs on real phones with SIMs and lands a forced change in about 3-5 seconds. Anyone promising per-second carrier rotation is selling the marketing, not the radio.