Somewhat a duplicate of Can a PCB trace support peak currents?
Are there rules of thumb for a PCB designer to compensate the output of via / trace width calculators to account for cases where the trace / via carries no current at all most of the time?
If it's easier to work from an example, let's assume that the circuit carries 20 amps when on, 0 when off, that the relevant part of the electronics is limited to a high-side driver, that the power line enters and exits the PCB via 18AWG wires soldered into properly sized vias, and that the high-side IC is 1/4" away from both of those vias. Power pulses last at most 5 seconds, and I can disable the high-side driver for as long as I want between pulses if needed - but the goal would be to allow at least 3 pulses anywhere in any 5 minutes timeframe. We can also assume that this is a standard 1oz, two layers PCB, in an airtight box, at room temperature.
I've searched for temperature vs time graphs for PCBs and found very little (there's one in figure 18 of this document, but the context is quite different).