Input RC filter design for analog pin of ADC
No matter how hard you try and convince yourself you have a dc input you'll be incorrect. If it were truly a dc input with no ac content then measuring the value once will be the same value as it is in 100 years time.

So, you have to consider how long it might take the signal to settle to "constant" when it is activated. If it is 1 second (say) then you need to consider that an RC filter having a time constant of 1 second is inappropriate. Most people assume the an RC filter settles to 99% in 5 time constants so in reality you might be looking at an RC filter with a cut-off of 5Hz: -



Maybe even ten time constants is more appropriate for your desired accuracy - this makes the filter a 10Hz low pass filter.