What is the LOW frequency resolution rule analogous to Nyquist?
The "limit" depends on the signal-to-noise ratio, and the nearest interfering frequency(s). 

If the nearest interfering frequency carrier is tuned 1% away, you might need (not just 6, but) 250 cycles or more to resolve the two separated frequency peaks, depending on the window and the S/N.  Whereas, a single pure unmodulated sinusoid in zero noise might be determined in as little as 3 or 4 non-aliased samples.

Also, there may be better low frequency estimators than depending on just a single non-rectangularly-windowed peak DFT magnitude bin, as using just that magnitude bin might be throwing some information about the phase of interference from the (nearby) conjugate negative frequency image that could be useful in improving frequency estimation and/or detection.