Resonant frequency:
Fr=1/(2*pi*sqrt(L*C))
Where Fr=resonant frequency in Hertz, pi=3.1415926, sqrt means square root, L=inductance in Henries, C=capacitance in Farads.
An ideal LC circuit with no series resistance and lossless inductance and capacitance will oscillate forever once current is injected into the circuit. This only happens in simulations, because there are no lossless inductors or capacitors in the real world (at room temperature). Real-world LC oscillators overcome the losses by adding an amplifier with positive feedback around the LC circuit.
If your simulation showed a decaying waveform, then you either had some resistance in your pulse source, or your models had losses built into them.