From oscilloscope tests it works rather well, the tricky part is matching the capacitor and any parallel discharge resistor to the impedance of the ADC. I found the AVR ADC pulled the capacitor down rapidly, so I had to remove the parallel discharge resistor completely.
It's simply an RC time constant. If the circuit is working correctly you need the resistor. The ADC input impedance/resistance is approximately 100 megohms (probably the same ballpark as a 'scope probe) but it's not necessarily a "resistor to ground" and it can't be counted-on to discharge the capacitor. An electrolytic capacitor will probably have enough leakage resistance to eventually discharge but I wouldn't count on it.
The charge-time depends on the effective output impedance of the op-amp, and the capacitor. If the capacitor is too big you might draw too much current and damage the op-amp.