A microwave oven 2,4GHz has a wavelength of 12.2 cm, water maser fundamental resonance is around 22GHz, and this is in 1/4 wave length 3,4mm... There are however many modes coming up to 96GHz witch would have a 1/2 wave around 1,5mm stans gap aproximatelly.
The lc is related i think to the spacial displacement of the ions, electrons... But i'm not sure on that... Stan stated so many things... seems more to be in accordance with the laser pulse repetition rate, commonly between 5khz and 10khz...
The time for transition of exited state to spontaneous emission is around 10^-8 seconds, so at least 100MHz frequency.. Lasers to get coherence needs to have more molecules in exited state, and is a matter of probability, so one never goes for the first transition to assure you get more atoms exited... To have stimulated emission you want all the atoms to emit in sync...
As i said water will emit laser but as its photons has low frequency of vibration, in the microwave infra-red range, its called maser. I think historically it came before the laser. Actually a laser is called an optical maser. Water Masers were observed in the universe...
Food for thought