It's not a simple resistance. There is a very small leakage current, which varies with temperature, and is not well specified. But that's not the main issue.
The input looks basically like a switch to a capacitor. When the ADC samples your signal, the switch connects, suddenly shorting the internal capacitance inside the chip to your signal. A current flows momentarily, to charge the capacitor to the same voltage (or close to the same voltage, if your signal has source impedance that causes a voltage drop between the true source and the internal ADC).
The internal capacitance is in the dozen pF range, and the switch on resistance is in the 1K range... approx.