Kirchoff's Laws and Maxwell Laws
If you want to use Kirchoffs laws you need to assume that the maximum wavelength of any signal that is relevant for the analysis is significantly greater in wavelength than your circuit board's maximum dimension (to avoid significant error). Signal wavelength examples in free-space: -

300 MHz has a \$\lambda\$ of 1 metre
30 MHz has a \$\lambda\$ of 10 metre
3 MHz has a \$\lambda\$ of 100 metre

As a signal on your circuit board and due to the PCB material, you should assume that \$\lambda\$ may be as low as half the free-space values quoted above.