Current Transformer Parameters

You can arrive at an estimate by first calculating the inductance of the secondary. This may be on your data sheet, or it can be calculated from the number of turns, turn area, and core material. From there, the high pass response will depend on the input impedance to the amplifier you are using to buffer / amplify the sense signal off the secondary or the termination resistance (whichever is lower... probably the termination resistance). Specifically, it should be fl=Rterm/(2*pi*Lsecondary).

I am using 20:1 A CT with winding ratio 1:2500.

Not quite. You are using a 2500:1 transformer and this transforms a current of 1 amp in the primary to a current of 0.4mA in the secondary (providing you have an appropriate load).

I am interested in RMS value

If you are measuring load current of some AC appliance then the current is likely not to be very sinusoidal and the only realistic way of determining true RMS is to sample the output waveform at many times the AC power frequency.

The CT's output can be offset and biased so that the full waveform (at max current) is within the upper and lower bounds of your PIC's ADC. You then sample this at (say) 1kHz, square each sample, accumulate over a period (say 1 second) and then take the square root of the accumulated 1 second value.

If you don't want to go to that trouble, you can usually place a diode in series with the burden resistor to get a rectified half wave output (also add a clamping diode for the negative half cycle of the CT's output). You can also use a bridge rectifier between burden and CT but because there are two diode drops you need to check the data sheet to see if the extra volt-drop from two diodes will cause inaccuracies. This can of course be easily filtered into a relatively smooth DC value before feeding into your PIC's ADC input. It won't be exactly true-RMS but may be good enough for your needs.