(i.e. one full 38kHz wavelength)?
Wavelength is the wrong thing here.
Period is how long it takes for one cycle, that is the signal to go high to low to just high again. Frequency is the number of times it does this in a second. 1 / the period gives the frequency and 1 / the frequency gives the period.
So for 38 KHz the period is 26.3uS, that will be 13.15 uS high and 13.15 uS low.