Measuring the input Cap discharging is not the greatest way to do it. By the time you've detected a significant voltage drop to determine you've a power failure (taking into account the dips and peaks of your various loads), that failure has been around perhaps for several tens of milliseconds.
Consider the attached schematic. Here the failure is detected the moment it happens. C1 has not yet started to discharge and you've got those extra tens of mS to play with... one extra Diode is all it takes...
R1/R2 are just a voltage divider to ensure the PIC's ADC doesn't get more than +5v at it's input. Adjust the values to suit.
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