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Calibration (offset and full scale) of AD7799 sequence and period question

I am going to use AD7799 in a project to measure two different analog inputs.

I need to convert two channels every 15 secs. so every 15 secs: I run a single conversion for CH1 and then change Config register to set input channel CH2 then run a single conversion.

This procedure continues for a long time (years maybe).

I want to know when I need to run internal zero scale and full scale calibrations ?!

should I run the both calibrations (zero scale and full scale) each time I change the channel ? then I need to run calibrations every 15secs for each channel ... ?

If so, there would be a problem, I need to spend time for 2 calibrations for each channel every 15 secs, which is not good at all ! the board is battery powered and power consumption is so important for me, so I should minimize the time of ADC conversion running ... (which would be much increased if I run calibration each time).

Can I run calibration for each channel once at first power up and then store the values in MCU (by reading the offset and full-scale registers), then I write it to AD7799 offset and full-scale registers for each channel (write saved values for CH1, then run CH1 single conversion, write saved values for CH2, then run CH2 single conversion) ? so no need to spend time for calibration conversions each time.

however if I do so, calibration is ran only once at power up for example and that saved value would be written in offset and full-scale register for next times (up to years maybe !).

I guess better is that the calibration values be updated after some time ? maybe I update the Calibrations values every 24 hours ? (still not every 15 Secs)

Which procedure do you recommend for handling this matter (considering power consumption) ?

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