To be strictly accurate the input impedance is 14pF (the sample and hold capacitor).... There are internal series resistances of 1k to 100k before that (yes, the datasheet has a value with 2 decades of uncertainty! fig 23-8 in datasheet). So source impedances of around 100k or more will definitely start to increase the settling time (which is 10us worst case by my calculations).
The sample and hold freezes 1.5 ADC clocks after the multiplexer updates, and the default Arduino setting is an ADC clock period of 8us. Thus there is 12us settling time available, fine for low impedance sources (25k or less - though if the internal series resistance is more like 1k than 100k a source of 120k would do OK). For a source of 1M 100us is needed (so two successive calls to analogRead () will be enough.
For more than 1M you have to scale the settling times by 100us per 1M.
An ADC conversion takes 13 ADC clocks (104us in default setup)