There are two different data freshness rates or time depending on the lower frequency you want to measure.
Here is the cut/paste from the 'Specifications' tab for that module;
2 Hz - 500 kHz at 1.0 s Data Freshness
20 Hz - 500 kHz at 0.1 s Data Freshness
So, if you want to measure the lower frequency, you will need to give the module longer to measure the signal (since the signal is slower).