ADC Calibration frequency

The calibration routine run by the boot ROM is located in TI reserved OTP, and just copies cal data from TI OTP to the appropriate ADC registers.  This trims the gain and offset to within the datasheet specifications.  You can do better than this by doing additional calibration, but that does not mean just running the same cal routine over again.  That won't do anything.  Rather, you need to perform offset trim yourself (i.e., convert a small voltage and determine the offset error) and then trim the offset registers.  Beyond that, you can do two-point calibration to determine the gain error, but the adjustment for gain error must be done in software after each conversion result is read (offset error has a hardware trim, but gain error does not).

The factory ADC cal (built-in cal routine) does not account for errors introduced by your input signal conditioning (i.e., op-amps, sensors, etc.).  The board level errors are what the manual calibration handles.  The built-in cal routine only handles device level errors.

In terms of how often you need to do this manual calibration, basically it would be just once unless you have board level errors that change with time, temperature, changing voltages in your system, etc.

Regards,

David