LTSpice: How to use .meas command to find the fundamental frequency of non-sinusoidal signal?
It looks like you know your final simulation time, and the waveforms are about settled at the end of it, so, considering what I see in your picture, you can consider 4.4ms a mark from which you can measure, like this:

.meas t1 find time when v(out)=0 cross=1 td=4.4m
.meas t2 find time when v(out)=0 cross=3 td=4.4m
.meas T param t2-t1




Looking more closely (the grey-ish waveform), it looks like you have skewed sines in there, which will account for harmonics, so you can't really rely on this measurement for detecting the fundamental, only its apparent period. Just thought I should add this.