How to calculate opamp input noise for DC input?
TI's Analog Applications Journal includes an article about noise analysis for an op amp driving an ADC as in your application. The author uses \$F_{\text{L}} = 0.1\text{Hz}\$ with the following rationale:


  When we think about noise at these low frequencies, we may jump to the conclusion that we should take this formula down to a very low frequency, such as 0.0001 Hz (0.0001 Hz = 1 cycle per 2.8 hours). However, at frequencies
  lower than 0.1 Hz, which is one cycle every 10 seconds, it is very possible that other things such as temperature, aging, or component life are changing in the circuit. Realistically, low-frequency noise from the amplifier will probably
  not appear at this sample speed; but changes in the circuit, such as temperature or power supply voltage, may.


Additionally, the OPA625 datasheet specifies the input noise voltage starting at \$f = 0.1\text{Hz}\$.

\$F_{\text{L}} = 0.1\text{Hz}\$ seems like a good choice since all three documents use it.